Journal

Journal Articles


*corresponding author

Submitted / Under Review

53. "A Bouncing and Rotating drop after Oblique Impact on Lubricant-Impregnated Surfaces," C. Bae, Y.-S. Ko, S. Shin, C. Lee*, Physics of Fluids (Accepted)

52. "Highly Porous Hydrogel for Efficient Solar Water Evaporation", A. R. Pati, Y. S. Ko, C. Bae, I. Choi, Y. J. Heo*, C. Lee*, Soft Matter 20, 4988-4997 (2024)  (link)

Solar energy is a plentiful renewable resource on Earth, with versatile applications in both domestic and industrial settings, particularly in solar steam generation (SSG). However, current SSG processes encounter challenges such as low efficiency and the requirement for extremely high concentrations of solar irradiation. Interfacial evaporation technology has emerged as a solution to these issues, offering improved solar performance compared to conventional SSG processes. Nonetheless, its implementation introduces additional complexities and costs to system construction. In this study, we present the development of a hydrophilic, three-dimensional network-structured hydrogel with high porosity and swelling ratio using a facile fabrication technique. We systematically varied the mixing ratios of four key ingredients (polyethylene glycol diacrylate, PEGDA; polyethylene glycol methyl-ether acrylate, PEGMA; Phosphate-buffered saline, PBS; and 2-hydroxy-2-methylpropiophenone, PI) to control the mean pore size and swelling ratio of the hydrogel. Additionally, plasmonic gold nanoparticles were incorporated into the hydrogel using a novel methodology to enhance solar light absorption and subsequent evaporation efficiency. The resulting material exhibited a remarkable solar efficiency of 77% and an evaporation rate of 1.6 kg/m2 h under standard solar illumination (one sun), comparable to state-of-the-art SSG devices. This high efficiency can be attributed to the synergistic effects of the hydrogel's unique composition and nanoparticle concentration. These findings offer a promising avenue for the development of highly efficient solar-powered evaporation applications. 

51. "Dynamics of Microdroplet Generation via Drop Impact on a Superhydrophobic Micropore", M. S. Reza, Y. S. Ko, B. E. Jeon, P. Sen, C. Lee*, Physics of Fluids 36, 052013 (2024) (link) selected as Editor's pick

This study delves into the dynamics of generating microdroplets by impacting a droplet onto a micropore on superhydrophobic copper substrates. It identifies the necessary impact velocities for single microdroplet formation for each micropore and characterizes microdroplet size in relation to micropore diameter. The results underscore the significant role of viscosity, especially as the diameter of the micropore decreases. For micropores measuring 400 μm, an increase in viscosity up to 8 cP does not alter the critical impact velocities, while smaller diameters of 50 and 100 μm see a notable change in critical velocities with even minor increases in viscosity. Remarkably, the diameter of the microdroplet remains consistent regardless of changes in the liquid viscosity or impact velocity. This research showcases two practical uses of single microdroplets: printing on paper and fabricating microbeads. The insights gained from these findings pave the way for advancements in printing technology and microfabrication techniques.

50. "High-powered superhydrophobic pyroelectric generator via droplet impact", J. Han, S. Shin, S. Oh, H. J. Hwang, D. Choi, C. Lee*, Y. Nam*, Nano Energy 126, 109682 (2024) (Link)

Recent studies on water-based pyroelectric generators (PEGs), which convert thermal energy to electrical energy, have focused on different operational modes like water evaporation, water stream, and droplet sliding. However, the development of sustainable, high-powered generators and comprehensive theoretical models has been limited. In response, our research introduces a droplet-based superhydrophobic pyroelectric generator (S-DPEG), exploiting the characteristics of lead magnesium niobate-lead titanate (PMN-0.3PT) coated with titanium dioxide nanoparticles. We analyzed power density by considering the phase transient temperature that maximizes the pyroelectric coefficient of PMN-0.3PT, testing various Weber numbers and droplet diameters within a moderate operating temperature range of 40°C to 80°C. Considering the dynamic characteristics of water droplet on superhydrophobic surfaces, we suggest a peak current model that can accurately predict the peak current within ~15% of error. Also, the maximum power density of 54.5 μW/cm2 at a droplet diameter of 3.6 mm and a temperature of 80°C, a noteworthy improvement over 3 times higher than previous water-based PEGs. Our results enhance the understanding of the pyroelectric effect coupled with drop impact dynamics and outline novel strategies for designing high-performance water-based PEGs.

49. "Graphene Oxide-based Nanofluidic System for Power Generation from Salinity Difference", Y. S. Ko, H. Cho, J. Han, Y. Nam*, S. Kim*, C. Lee*, Journal of Membrane Science 701, 122722 (2024) (Link)

In converting a salinity difference to the electrical power by using an ion-selective membrane, achieving a high-power density necessitates both a high ion permeability and ion selectivity of the membrane. However, meeting these two requirements often leads to the conflicting tradeoff in the membrane properties. In this study, we introduce a new mechanistic approach to meeting both requirements by combining an ultra-thin (<100 nm thickness) graphene oxide-based membrane for a high permeability with an asymmetric access area for a high ion selectivity, forming a new type of ionic-diode nanofluidic system. With a graphene oxide/silk fibroin composite membrane, a large power density of 2kW/m2  is achieved with 32% conversion efficiency under a 1000-fold salt concentration ratio. This approach can be utilized to overcome the low power density limitation with any ultra-thin membranes, and thereby it will provide a new route to utilize blue energy in a reliable and efficient way. 

48. "Facile fabrication of superhydrophobic sub-millimetric cone-shape pillars based on a single UV exposure to control drop impact dynamics ", Y. S. Ko, C. Ha, Y. J. Heo, C. Lee*, Journal of Mechanical Science and Technology 38(8), 4255-4260 (2024) (Link) selected as Editor's pick

When a water drop is impinged upon a superhydrophobic surface with sub- millimetric surface structures, unconventional impact dynamics such as pancake-like bouncing and asymmetric drop spreading can occur. However, the fabrication of such surface structures often requires an unconventional fabrication approach, which is either time-consuming or costly. In this study, we propose a simple lithography-based approach to manufacture sub- millimetric cone-shaped pillars over a large area by taking advantage of a unique optical prop- erty of hydrogels: a change of refractive index after UV-curing. With an additional hydrophobic nanoparticle coating, we demonstrate that such structures can be used to reduce the contact time during drop impact and induce a drop rotation during rebound. Moreover, the flexibility of hydrogels enables the transfer of surface structures to non-planar substrates.

47. "Effect of superhydrophilic surface on the cavitation behaviors of rotating blades", H. Choi, S. Oh, C. Lee, H. Choi, H. Park*, Physics of Fluids 35, 113316 (2023) (Link)

We experimentally confirmed the idea of mitigating (or delaying) the cavitation on the turbomachinery (rotating blades) by transforming the blade surface to be superhydrophilic, thereby the population of the cavitation nuclei is reduced near the surface. We focused on the changes in the cavitation incidence rate, amount of cavitation bubble, and bubble distribution on the superhydrophilic blade through the high-speed camera imaging, compared to the case with a regular (i.e., smooth) surface. With superhydrophilic blades, the cavitation incidence rate decreased significantly, indicating that fewer nuclei evolved into the actual cavitation bubbles. This is also associated with 8.6% delay of the critical rotational speed at which the cavitation process is almost completely established (incidence rate exceeds 80%), and the reduction in the total amount of cavitation bubbles was achieved as much as 18% (maximum 38% in the tested range of rotational Reynolds number). Additionally, the distribution of cavitation bubbles was generally pushed upstream, with fewer bubbles extending downstream, i.e., pushed away from the blade trailing edge. We believe the present results are promising enough to spur the follow-up investigation for the in-depth analysis and practical application toward the robust cavitation control without the substantial modulation of the geometry.

46. "Numerical study of rectified electroosmotic flow in nanofluidics: Influence of surface charge and geometrical asymmetry", T. D. Mai, C. Lee*, J. Ryu*, Physics of Fluids 35, 092017 (2023) (Link)

The transport of ions in nanofluidic systems, specifically the rectified ion transport or the ionic diode phenomenon occurring in the presence of asymmetrical geometry and/or charge distribution, has drawn considerable attention due to its relevance in energy conversion and biosensing applications. However, previous numerical research has frequently overlooked the concurrent liquid flow within these systems, even though multiple experimental studies have highlighted intriguing flow patterns in ionic diode configurations. In the present study, we employ comprehensive numerical simulations to probe the influence of geometrical or charge asymmetry in a nanofluidic system on electroosmotic flow and ion transport. These simulations employ the Poisson–Nernst–Planck equation in conjunction with the Navier–Stokes equation. Our findings reveal that even when the current rectification trend is consistent between conical and straight nanopores, charge asymmetry and geometric asymmetry can generate significant variations in the rectification effects of electroosmotic flow. Furthermore, our research indicates that the direction of ion rectification and flow rectification can be independently manipulated by utilizing charge asymmetry in conjunction with geometric asymmetry, thereby facilitating advanced control of ions and flows within nanofluidic systems. Collectively, our findings contribute to a more profound understanding of the mechanisms underlying osmotic flow rectification and propose a novel approach for developing efficient ion and flow rectification systems.

45. "Passive control of flow rate change due to the input pressure fluctuation based on microchannel deformation", M. S. Nam, H. T. Sang, H. G. Choi, K. W. Kim, C. Lee*, Y. J. Heo*, Physics of Fluids 35, 102014 (2023) (Link)

Precise and controlled drug delivery is crucial in continuous infusion systems used for drug treatment, anesthesia, cancer chemotherapy, and pain management. Elastometric pumps are commonly utilized in continuous infusion systems for their ease of use and cost-effectiveness. However, the infusion accuracy is often compromised due to the fluctuating supply pressure of elastomeric pumps, requiring an additional flow regulator to stabilize the output flow rate. We, here, present a novel approach to passively control a flow rate even under the fluctuating pressure environment based on a channel deformation. The flow rate control is enabled by a flow regulator consisting of an open-end microchannel, a closed-end microchannel, and a flexible membrane in the middle. The pressure within an open-end microchannel decreases in the downstream direction, while the pressure within a closed-end microchannel remains equal to the input pressure, creating the pressure difference between the two channels. The membrane deforms in response to this pressure difference, allowing for adjustment of the output flow rate by decreasing the flow path area with the increase in the input pressure. It is found that this concept successfully works by maintaining a steady output flow rate over a target pressure range of 40–50 kPa. Fluid–structure interaction numerical simulations and theoretical analysis are used to explain the flow rate control mechanism of the device. The results show that the present approach offers a promising solution for achieving stable drug delivery in continuous drug infusion systems, addressing the limitations of conventional elastomeric pumps.

44. "Depressurization-induced drop breakup through bubble growth",C. Pirat*, C. Cottin-Bizonne, C. Lee, S. M. M. Ramos, O. Pierre-Louis, Physical Review Fluids 8, L091601 (2023) (Link)

Drop breakup is often associated with violent impacts onto targets or fast inner bubble growth consecutive to phase change. We report on a well-controlled drop breakup experiment where bubble growth is triggered by the decrease of the ambient pressure. The drop initially sits on a textured hydrophobic surface at controlled temperature, and a bubble grows from the center of the liquid-solid interface. We find a transition from top-breakup to triple-line breakup depending on the initial contact angle of the drop. A minimal model based on inertial dynamics and constant bubble pressure is proposed. It quantitatively captures the growth of the bubble and the distinction between top or triple-line breakup. However, the model only provides an upper bound for the breakup time.

43. "Enhanced Capillary and Heat Transfer Performance of Asymmetric Micropost Wicks", S. Bang, J. Kim, S. Ryu, S. Ki, Y. J. Heo*, C. Lee*, Y. Nam*, International Communications in Heat and Mass Transfer  146, 106935 (2023) (Link)

We developed asymmetric capillary wicks composed of slanted microposts using inclined photolithography. Then we investigated the effects of inclination angle and wicking direction on the capillary and the heat transfer performances. The working fluid accelerates when it flows in the slope direction of the structure (forward direction, FD) and decelerates when it flows in the opposite slope direction (rear direction, RD). We applied the scaling law to the capillary rise experiment data to verify that the inclination angle and the wicking direction affect the capillary performance. The capillary performance parameter was improved by up to ∼39% with FD case and decreased by 21.3% with RD. The heat transfer performance test showed that the wick-CHF (the enhanced critical heat flux due to the formation of the wick) of the asymmetric FD case was increased by 43.3% compared to symmetric ones while maintaining the heat transfer coefficient. This work shows that asymmetric evaporator wicks can enhance the critical heat flux without sacrificing the heat transfer coefficient, which can help develop high-performance thermal management solutions.

42. "Thermally enhanced osmotic power generation from salinity difference", J. Han, Y. S. Ko, Y. Nam*, C. Lee*, Journal of Membrane Science 672, 121451 (2023) (Link)

Recently, membrane-based power generation from salinity difference has been in the spotlight as a blue energy harvesting, but achieving a high power density and conversion efficiency still remains as a major challenge in this approach. Instead of developing new membranes, regulating the thermal condition within the membrane has been proposed as a way to enhance the power generation by several numerical studies, but this concept has rarely been explored through the systematic experimental studies due to the difficulty of imposing a controlled temperature gradient within the membrane. In this work, we experimentally and systematically study how the temperature difference can influence osmotic power generation using a commercial polycarbonate membrane and demonstrate that even when a thermal gradient is negligibly small within the nanoporous membrane, it is still possible to achieve a significant enhancement of the power generation. We propose that the effective ion concentration at the interfacial region between the reservoir and the membrane varies with the direction of the imposed temperature difference, such that the opposite direction of salinity and temperature differences can lead up to 5.3 times power enhancement as a result of the increase of the effective ion concentration ratio across the membrane. As an example of practical applications, we apply our findings to a floating type nanogenerator by incorporating a solar absorber to generate the temperature difference spontaneously under solar radiation conditions, and the results with the nanogenerator show that the power generation is indeed enhanced under both simulated and actual solar radiation conditions. We believe that our approach can be applied to any nanoporous membrane regardless of its thermal property, and therefore would provide a practical path to the power enhancement of reverse electrodialysis systems. 

41. "Plastron replenishment on superhydrophobic surfaces using bubble injection", H. Sung, H. Choi, C. Ha, C. Lee, H. Park*, Physics of Fluids 34, 103323 (2022) (Link)

While the air lubrication by bubble injection and superhydrophobic (SHPo) surfaces have been investigated vigorously for flow control, e.g., underwater drag reduction, further advancement seems to be delayed. For the former, large air flow rate is required for the meaningful performance and furthermore the injected bubbles do not stay over the surface willingly. Depletion (diffusion) of the trapped air pockets on the SHPo surface is a critical issue for the latter. In the present water-tunnel experiments, we show that the above-mentioned challenges can be successfully overcome by combining the two methods, i.e., the plastron on SHPo surfaces can be replenished in turbulent flows with a very small amount of air, even after the surface is fully wetted. To analyze the phenomena, the bubble-plastron interaction is visualized and quantified while introducing bubbles over the SHPo surfaces (with random roughness or longitudinal grooves) in the turbulent boundary layer flow of ReL = 0.3 − 1.1 × 106 . The plastron on SHPo surfaces with longitudinal grooves is retained in a film-like shape with a quite smaller amount of air than that with random roughness. By quantifying the light intensity from the surface, we suggest a scaling relation between the effective plastron thickness and surface light intensity, which would serve as a criterion for the successful plastron replenishment. Finally, the morphology of the plastron is classified into different regimes, depending on the Reynolds number, air flow rate, and surface roughness types. 

40. "Promoting rebound from droplet impact on a spherical particle: Experimental and numerical study", I. Yoon, C. Ha, C. Lee, S. Shin*, Physics of Fluids  34, 103302 (2022) (Link) 

In this study, we experimentally and numerically investigate the activity of a rebounding droplet on a spherical particle and the effects of surface curvature on its rebounding behavior. We report that the rebound of the droplet can be promoted in smaller particles. As the droplet-to-particle size ratio increases, the critical Weber number is significantly reduced, and the restitution coefficient is much increased. The underlying physical mechanism for the promotion of the rebound is the reduced energy dissipation on the smaller particles in the very early stages of the collision, and this reduction mainly occurs as the liquid is being squeezed. This reduced energy loss allows larger liquid–gas interfacial deformation at the maximum spreading state and also allows more drastic retractions during the recoiling stage, which eventually leads to the promotion of the rebound. 

39. "Influence of early drop bouncing on heat transfer during drop impact", Y.S. Ko, J. Kim, S. Ryu, J. Han, Y. Nam*, C. Lee*, International Communications in Heat and Mass Transfer 137, 106235 (2022) (Link)

Reducing the contact time with the water drop during impact has attracted much attention as a way to minimize the thermal interaction with water. Although various types of superhydrophobic surfaces have been developed for contact time reduction, few works investigated the actual heat transfer when the contact time is reduced. Here, by using all metallic superhydrophobic surfaces, we experimentally study the influence of pancake-like early bouncing on the change of surface temperature during drop impact. First, we propose the design criteria for early bouncing on mesh surfaces as a function of a mesh geometry and an impact velocity. Then, the thermal interaction during drop impact is quantified by recording the maximum temperature drop on the surfaces when the cold water drop is impinged onto the surface. The results show that the heat transfer is reduced on all tested mesh surfaces over the flat superhydrophobic plate, but unexpectedly the temperature change is not correlated with the contact time reduction. We propose that the inevitable contact area increase associated with early bouncing can counterbalance the effect of contact time reduction, which highlights the importance of the consideration of both contact time and contact area for heat transfer during drop impact.

38. "Organic/Inorganic Hybrid Cerium Oxide-based Superhydrophobic Surface with Enhanced Weather Resistance and Self-recovery", S. Oh, J. Shim, D. Seo, M. J. Shim, S. C. Han, C. Lee*, Y. Nam*, Progress in Organic Coatings 170, 106998 (2022) (Link)

For a superhydrophobic coating, its limited durability has been a persistent issue that prevents its widespread usage in outdoor applications. Here, we propose a scalable, self-recoverable CeO2/PDMS hybrid coating that harnesses synergetic benefits from hydrocarbon adsorption of rare earth oxides and hydrocarbon supply by a hydrocarbon-based polymer. It is demonstrated that this hybrid coating substantially outperforms other super-hydrophobic surfaces in self-recovery of superhydrophobicity and weather resistance. The synergetic effect ex-pedites the recovery of superhydrophobicity via a facilitated hydrocarbon adsorption: e.g., the self-recovery time of our coating was over 30 times less than that with CeO2 nanoparticle-based coating after plasma treatment. Furthermore, our coating showed excellent weather resistance by (1) sustaining superhydrophobicity over 1 year without any deterioration in the outdoor environment and (2) surviving accelerated weathering tests. Finally, our coating was successfully applied to the outdoor electrical insulators, while exhibiting excellent self-recovery performance of superhydrophobicity even after exposure to 600 V of electrical stress in presence of conductive water droplets. We believe that our coating provides robust superhydrophobicity via a rapid self-recovery performance and can be applied to any type of substrates with complex geometry by a one-step spraying process, both of which would be crucial to the application of the superhydrophobic coating in a wide range of energy and environmental applications.

37. "Reducing Surface Fouling Against Emulsified Oils Using CuO Nanostructured Surfaces", S. Oh, J. Lee, D. Seo, M. C. Shin., J. K. Lee, C. Lee*, Y. Nam*, Colloids and Surfaces A: Physicochemical and Engineering Aspects  612, 125991  (2021) (Link)

Surface fouling by oil is a significant engineering issue by degrading the performance of various energy systems. Recently, it has been shown that nanostructured surfaces with the proper wettability can mitigate surface fouling against various types of organic or inorganic matter in aqueous environments. However, their effectiveness in suppressing surface fouling is questionable against low surface tension bio-oils, particularly when they are present in the form of emulsified oils. Here, we show that a surface fouling on the metallic substrate can be mitigated by nanostructuring the substrate, followed by additional surface treatment. With hydrophobization of nanostructured substrate, oil-fouling is reduced up to ∼ 48 % due to the reduced surface energy, although emulsified oil still sticks to the surface. Furthermore, with additional infusion of low surface tension lubricants, oil-fouling is significantly suppressed up to ∼ 88 % due to non-sticking property of the lubricant-infused substrate even to emulsified oils. Also, it is found that surface fouling is strongly dependent on the temperature due to the change of emulsion property with the temperature, which should be taken into account in practical settings. By proposing a practical solution to minimizing surface fouling by emulsified bio-oils, we believe that our results can help address a critical fouling issue in energy systems utilizing bio-oils. 

36. "Quantifying frictional drag reduction properties of superhydrophobic metal oxide nanostructures", Y. S. Ko, H. J. Kim, C. Ha, C. Lee*,  Langmuir 36(40), 11809-11816 (2020) (Link)

We measure the frictional drag-reducing property of various superhydrophobic metal oxide nanostructures by quantifying their effective slip length. Scalable chemical methods tailored to each metal substrate are applied to grow oxide nanostructures on copper (Cu), aluminum (Al), and titanium (Ti), respectively. In particular, three different types of oxide nanostructures are grown on the titanium substrate by changing the chemical composition to investigate the morphological influence on the slip length. Microchannels containing metal oxide nanostructures are fabricated based on the microfluidic sticker method, while the slip length is unambiguously determined by measuring the ratio of the volume flow rate over the superhydrophobic surface to that over the flat surface simultaneously. The slip length is measured to be 6.8 ± 1.4 μm on Cu nanostructures, while it is measured to be 2.5 ± 0.6 μm on Al nanostructures. For Ti nanostructures, the measured slip lengths range from 1 to 2.5 ± 0.5 μm, where they increase proportionally with the structural pitch of the nanostructures, agreeing with the theoretical predictions. We believe that our results will be useful in applying scalable low-cost metal oxide nanostructures to underwater applications by providing their frictional characteristics. 

35. "Endowing anti-fouling properties to metal substrates by creating artificial barrier layer based on scalable metal oxide nanostructures", K. Song, J. Shim, J.-Y. Jung*,  C. Lee*, Y. Nam*, Biofouling 36(7), 766-782 (2020) (Link)

Here, by creating different types of artificial barrier layer against bacterial attachment, anti-biofouling properties were endowed on three metallic surfaces–aluminum, stainless steel and titanium. To each metallic surface, a tailored chemical oxidation process was applied to grow scalable oxide structures with an additional appropriate coating, resulting in three different types of anti-biofouling barrier, a thin water film, an air layer and an oil layer. Fluorescence images of the attached bacteria showed that the water layer improved the anti-biofouling performance up to 8–12 h and the air layer up to 12–24 h, comparable with the lifetime of the air layer. In comparison, the oil layer exhibited the best anti-biofouling performance by suppressing the fouled area by < 10% up to 72 h regardless of the substratum type. The present work provides simple, low-cost, scalable strategies to enhance the anti-biofouling performance of industrially important metallic surfaces.

34. "Water penetration dynamics through Janus mesh during drop impact", C. Bae, S. Oh, J. Han, Y. Nam*, C. Lee*, Soft Matter 16(26), 6072-6081 (2020) (Link)

Here, we study the water penetration dynamics through a Janus membrane with opposite wettability, i.e., (super-) hydrophobic on one side and (super-) hydrophilic on the other side, during drop impact. It is demonstrated that the penetration dynamics through the membrane consists of two temporally distinct events: dynamic pressure driven penetration dynamics on a shorter timescale and capillary pressure driven penetration dynamics on a longer timescale. For penetration under dynamic pressure, the threshold velocity for the penetration is dependent on the wettability of the impact side, such that a smaller impact velocity is required for water penetration when a water drop is impinged onto the superhydrophobic side over the superhydrophilic side. We demonstrate that this difference in the penetration dynamics upon drop impact can still be accounted for by the balance between the dynamic pressure and the capillarity pressure after adjusting the relative magnitude of the two contrasting pressures required for the penetration. Meanwhile, it is demonstrated that the penetration dynamics under capillary pressure is governed by the balance between the capillary pressure and the viscous pressure while the penetration mainly proceeds through the penetration area, which is formed during short-time penetration, showing the dynamic coupling between the two penetration dynamics. By elucidating the penetration dynamics on a Janus membrane, we believe that our results can help in designing Janus membranes for various fluidic applications such as oil–water separation, aeration, and water harvesting.

33. "Contact time on curved superhydrophobic surfaces", J. Han, W. Kim, C. Bae, D. Lee, S. Shin, Y. Nam*, C. Lee*, Physical Review E 101(4), 043108 (2020)  (Link)

When a water drop impinges on a flat superhydrophobic surface, it bounces off the surface after a certain dwelling time, which is determined by the Rayleigh inertial-capillary timescale. Recent works have demonstrated that this dwelling time (i.e., contact time) is modified on curved superhydrophobic surfaces, as the drop asymmetrically spreads over the surface. However, the contact time on the curved surfaces still remains poorly understood, while no successful physical model for the contact time has been proposed. Here, we propose that the asymmetric spreading on the curved surface is driven by either the Coanda effect or inertia depending on the ratio of the drop diameter to the curvature diameter. Then, based on scaling analysis, we develop the contact time model that successfully predicts the contact time measured under a wide range of experiment conditions such as different impact velocities and curvature diameters.We believe that our results illuminate the underlying mechanism for the asymmetric spreading over the curved surface, while the proposed contact time model can be utilized for the design of superhydrophobic surfaces for various thermal applications, where the thermal exchange between the surface and the water drop occurs via a direct physical contact.

32. "Drag reduction on drop during impact on multiscale superhydrophobic surfaces",  G. Martouzet, C. Lee, C. Pirat, C. Ybert, A.-L. Biance*, Journal of Fluid Mechanics (Rapids) 892, R2 (2020) (Link) 

(Focus on Fluids: A. Gauthier, Slippery bounces 896, F1 (2020)) 

Liquid drop impact dynamics depends on the liquid–substrate interaction. In particular, when liquid–solid friction is decreased, the spreading of the impacting drop lasts longer. We characterise this effect by using two types of superhydrophobic surfaces, with similar wetting properties but different friction coefficients. It is found that, for large enough impact velocities, a reduced friction delays the buildup of a viscous boundary layer, and leads to an increase of the time required to reach the maximal radius of the impacting drop. An asymptotic analysis is carried out to quantify this effect, and agrees well with the experimental findings. Interestingly, this novel description complements the general picture of drop impact on solid surfaces, and more generally addresses the issue of drag reduction in the presence of slippage for non-stationary flows.

31. "Brushed lubricant-impregnated surfaces (BLIS) for long-lasting high condensation heat transfer", D. Seo, J. Shim, C. Lee*, Y. Nam*, Scientific Reports 10, 2959 (2020) (Link) 

Recently, lubricant-impregnated surfaces (LIS) have emerged as a promising condenser surface by facilitating the removal of condensates from the surface. However, LIS has the critical limitation in that lubricant oil is depleted along with the removal of condensates. Such oil depletion is significantly aggravated under high condensation heat transfer. Here we propose a brushed LIS (BLIS) that can allow the application of LIS under high condensation heat transfer indefinitely by overcoming the previous oil depletion limit. In BLIS, a brush replenishes the depleted oil via physical contact with the rotational tube, while oil is continuously supplied to the brush by capillarity. In addition, BLIS helps enhance heat transfer performance with additional route to droplet removal by brush sweeping. By applying BLIS, we maintain the stable dropwise condensation mode for > 48 hours under high supersaturation levels along with up to 61% heat transfer enhancement compared to hydrophobic surfaces.

30. "Passive anti-flooding superhydrophobic surfaces", D. Seo, J. Shim, B. Moon, K. Lee, J. Lee, C. Lee*, Y. Nam*, ACS Applied Materials & Interfaces 12(3), 4068-4080 (2020) (Link) 

Superhydrophobic (SHPo) surfaces can provide high condensation heat transfer due to facilitated droplet removal. However, such high performance has been limited to low supersaturation conditions due to surface flooding. Here, we quantify flooding resistance defined as the rate of increase in the fraction of water-filled cavities with respect to the supersaturation level. Based on the quantitative understanding of surface flooding, we suggest effective anti-flooding strategies through tailoring the nanoscale coating heterogeneity and structure length scale. Experimental verification is conducted using CuO nanostructures having different length scales combined with hydrophobic coatings with different nanoscale heterogeneities. The proposed antiflooding SHPo can provide a ∼130% enhanced average heat transfer coefficient with ∼14% larger supersaturation range for droplet jumping compared to a previous CuO SHPo. The proposed anti-flooding parameter and the scalable SHPo will help develop high-performance condensers for real-world applications operating in a wide range of supersaturation levels.

29. "High-efficiency power generation in hyper-saline environment using conventional nanoporous membrane", J. Han, C. Bae, S. Chae, D. Choi, S. Lee, Y. Nam*,  C. Lee*, Electrochimica Acta 319, 366-374 (2019) (Link) 

Here we introduce the new approach to high-efficiency power generation from a salinity difference using conventional nanoporous Nafion membrane. When access areas on each side of nanoporous Nafion membrane are set to be asymmetric, the ratio of ionic current upon a voltage bias of the different polarity also becomes asymmetric, resulting in ionic diode phenomena. When this geometrical ionic diode effect is combined with a salinity gradient, it can help significantly improve the energy conversion efficiency from a salinity difference even under a hyper-saline environment with a large salinity difference, e.g. ~41% conversion efficiency and ~120 nW power generation with 1M KCl and 1000-fold salinity difference, both of which are comparable with the best performances reported in the previous studies. We propose that the decrease in ion concentration polarization at a low salt concentration side is responsible for the enhanced power generation with the membrane having asymmetric access areas. Our approach is simple to implement and can be applicable to any nanoporous membrane to enhance the power generation from a salinity difference.

28. "Continuous scavenging of broadband vibrations via omnipotent tandem triboelectric nanogenerators with cascade impact structure", D. Bhatia, H. J. Hwang, N. D. Huynh, S. Lee, C. Lee, Y. Nam, J.-G. Kim, D. Choi , Scientific Reports 9, 8223 (2019) (Link) 

Ambient vibration energy is highly irregular in force and frequency. Triboelectric nanogenerators (TENG) can convert ambient mechanical energy into useable electricity. In order to effectively convert irregular ambient vibrations into electricity, the TENG should be capable of reliably continuous operation despite variability in input forces and frequencies. In this study, we propose a tandem triboelectric nanogenerator with cascade impact structure (CIT-TENG) for continuously scavenging input vibrations with broadband frequencies. Based on resonance theory, four TENGs were explicitly designed to operate in tandem and cover a targeted frequency range of 0–40 Hz. However, due to the cascade impact structure of CIT-TENG, each TENG could produce output even under non-resonant conditions. We systematically studied the cascade impact dynamics of the CIT-TENG using finite element simulations and experiments to show how it enables continuous scavenging from 0–40 Hz even under low input accelerations of 0.2 G–0.5 G m/s2. Finally, we demonstrated that the CIT-TENG could not only scavenge broadband vibrations from a single source such as a car dashboard, but it could also scavenge very low frequency vibrations from water waves and very high frequency vibrations from air compressor machines. Thus, we showed that the CIT-TENG can be used in multiple applications without any need for redesign validating its use as an omnipotent vibration energy scavenger.

27. "Performance analysis of gravity-driven oil-water separation using membranes with special wettability", S. Oh, S. Ki, S. Ryu, M. C. Shin, J. Lee, C. Lee*, Y. Nam*, Langmuir 35, 7769-7782 (2019) (Link) 

A membrane with selective wettability to either oil or water has been utilized for highly efficient, environmentally friendly membrane-based oil−water separation. However, a predictive model, which can be used to evaluate the overall separation performance of the membrane, still needs further development. Herein, we investigate three separation performance parameters, that is, separation efficiency, liquid intrusion pressure, and mass flux in particular, as a function of pore geometry and liquid properties using metallic meshes whose surface wettability is modified by scalable spray coating. We show that the prepared membrane exhibits a separation efficiency over 98% below the intrusion pressure, while the intrusion pressure increases with the decrease of pore size of the membrane. Particularly, we develop a semi-empirical model for the mass flux through the membrane. As application examples of our performance analysis, we successfully predict the separation time for one-way and two-way gravity-driven separation of the oil−water mixture, the decrease of the mass flux due to membrane fouling, and the maximum allowable separation capacity of the given membrane. This work can help to design optimal membrane-based oil−water separation systems for actual industrial applications by providing a selection guideline for separation membranes.

26. "Influence of lubricant-mediated droplet coalescence on frosting delay on lubricant impregnated surfaces", D. Seo, S. Oh, B. Moon, H. Kim, J. Kim, C. Lee*, Y. Nam*, International Journal of Heat and Mass Transfer 128, 217-228 (2019) (Link) 

Condensation frosting causes serious economic and safety problems in many industrial applications. Recently, lubricant-impregnated surfaces (LIS) have been attracting much interest with their excellent anti-frosting ability. The facilitated removal of drops due to the low contact angle hysteresis of LIS has been suggested as the frosting suppression mechanism. Here, we demonstrate a hitherto-unexplored microscale frosting suppression mechanism on LIS by investigating microscopic condensation and freezing dynamics on LIS by varying the viscosities of the lubricants. Based on the ice propagation model, we show that the frosting propagation is suppressed on LIS with a low viscosity oil where the coalescence of droplets is promoted by the presence of oil. On the contrary, the coalescence between droplets is interrupted on LIS with a high viscosity oil, which facilitates the frost propagation. The criteria for the delay of condensation frosting were explained based on the competition between the lubricant drainage time and the drop growth time scale. Finally, we verify that microscopic frosting suppression mechanism of LIS persists up to macroscopic level by demonstrating that LIS is effective in suppressing condensation frosting on heat exchangers.

25. "Mesoporous highly-deformable composite polymer for a gapless triboelectric nanogenerator via a one-step metal oxidation process", H. J. Hwang, Y. Lee, C. Lee, Y. Nam, J. Park, D. Choi, D. Kim, Micromachines 9, 656 (2018) (Link) 

The oxidation of metal microparticles (MPs) in a polymer film yields a mesoporous highly-deformable composite polymer for enhancing performance and creating a gapless structure of triboelectric nanogenerators (TENGs). This is a one-step scalable synthesis for developing large-scale, cost-effective, and light-weight mesoporous polymer composites. We demonstrate mesoporous aluminum oxide (Al2O3) polydimethylsiloxane (PDMS) composites with a nano-flake structure on the surface of Al2O3 MPs in pores. The porosity of mesoporous Al2O3-PDMS films reaches 71.35% as the concentration of Al MPs increases to 15%. As a result, the film capacitance is enhanced 1.8 times, and TENG output performance is 6.67-times greater at 33.3 kPa and 4 Hz. The pressure sensitivity of 6.71 V/kPa and 0.18 A/kPa is determined under the pressure range of 5.5–33.3 kPa. Based on these structures, we apply mesoporous Al2O3-PDMS film to a gapless TENG structure and obtain a linear pressure sensitivity of 1.00 V/kPa and 0.02 A/kPa, respectively. Finally, we demonstrate self-powered safety cushion sensors for monitoring human sitting position by using gapless TENGs, which are developed with a large-scale and highly-deformable mesoporous Al2O3-PDMS film with dimensions of 6  5 pixels (33  27 cm2).

24. "Electron blocking layer-based interfacial design for highly-enhanced triboelectric nanogenerators", H.-W. Park, N. D. Huynh, W. Kim, C. Lee, Y. Nam, S. Lee, K.-B. Chung, D. Choi, Nano Energy  50, 9 (2018) (Link) 

The key to enhance the output power from triboelectric nanogenerators (TENGs) is to control the surface charge density of tribo-materials. In this study, we introduce an electron blocking layer (EBL) between a negative tribomaterial and an electrode to dramatically enhance the output power of TENGs. For the first time, we suggest that the tribo-potential can be significantly reduced by the presence of interfacial electrons; electrostatically induced positive charges at the interface beneath a negative tribo-material can be screened out by the electrons, thereby decreasing the surface charge density. By employing an EBL between a negative tribo-material and an electrode, we can maintain a high surface charge density at the surface of the negative tribo-material. Furthermore, an EBL with high permittivity can enhance the polarization of the tribo-material, resulting in an improved surface charge density. As a proof of concept, polydimethylsiloxane (PDMS) and aluminum (Al) are used as a negative tribo-material and an electrode, respectively. A TiOx EBL is then deposited in between these materials by radio frequency (RF) sputtering. Due to the coupling effects of the electron blocking and enhanced polarization, the output peak power from the TENG with a TiOx EBL reaches approximately 2.5mW at 3 Hz and 5 N, which is 25 times larger than that of a TENG without an EBL. To understand the improved behavior of the TENG with a TiOx EBL, we investigate the correlations between the output behavior of the TENG and the physical properties of the surface/interface of TiOx and PDMS (e.g., the surface potential, dielectric properties, and electronic structures). We expect that our results can provide a novel design way to significantly improve the output performance of TENGs.

23. "Anisotropic drop spreading on superhydrophobic grates during drop impact",  J. Han, S. Ryu, H. Kim, P. Sen, D. Choi, Y Nam*, C. Lee*, Soft Matter 14, 3760 (2018) (Link) 

We study the influence of geometric anisotropy of micro-grate structures on the spreading dynamics of water drops after impact. It is found that the maximal spreading diameter along the parallel direction to grates becomes larger than that along the transverse direction beyond a certain Weber number, while the extent of such an asymmetric spreading increases with the structural pitch of grates and Weber number. By employing grates covered with nanostructures, we exclude the possible influences coming from the Cassie-to-Wenzel transition and the circumferential contact angle variation on the spreading diameter. Then, based on a simplified energy balance model incorporating slip length, we propose that slip length selectively enhances the spreading diameter along the parallel direction, being responsible for the asymmetric drop spreading. We believe that our work will help better understand the role of microstructures in controlling the drop dynamics during impact, which has relevance to various engineering applications.

22. "Effect of Geometrical Parameters on Rebound of Impacting Droplets on Leaky Superhydrophobic Meshes", A. Kumar, A. Tripathy, Y. Nam, C. Lee*, P. Sen*, Soft Matter 14, 1571 (2018) (Link) 

When a droplet impacts a superhydrophobic sieve, a part of the droplet penetrates through it when the dynamic pressure of the impinging droplet exceeds the breakthrough pressure. At higher impact velocities, the ejected-jet breaks and separates from the main droplet. The remaining part of the droplet bounces off the surface showing different modes (normal bouncing as a vertically elongated drop or pancake bouncing). In this work, we have studied the effect of different geometrical parameters of superhydrophobic copper meshes on different modes of droplet rebound. We observe three different effects in our study. Firstly, we observe pancake like bouncing, which is attributed to the capillary energy of the rebounding interface formed after the breaking of the ejected-jet. Secondly, we observe leakage of the droplet volume and kinetic energy due to the breaking of the ejected-jet, which leads to reduction in the contact times. Finally, we observe that for flexible meshes, the transition to pancake type bouncing is induced at lower Weber numbers. Flexibility also leads to a reduction in the volume loss from the ejected-jet. This study will be helpful in the design of superhydrophobic meshes for use under impact scenarios.

21. "Scalable superhydrophobic flexible plasmonic poly(tetrafluoroethylene-co-perfluorovinyl ether) films via ion-beam irradiation and metal deposition", J. Jeon, S. Chae, D. Bhatia, C. Lee, Y. Nam, H. Kim, D. Choi, Materials Express 7(4), 319 (2017) (Link)

In this study, we report a method to produce large-area superhydrophobic plasmonic poly(tetrafluoroethyleneco- perfluorovinyl ether) (PFA) films using room temperature and high-throughput processes, such as ion-beam irradiation and thermal evaporation. Ion-beam irradiation on large-area PFA films changes their surface wettability from hydrophobic to superhydrophobic, as nano-size pores are gradually formed on their surfaces according to the ion beam density. Following irradiation gold evaporation creates plasmonic characteristics on the nanoporous PFA films. Although gold films normally show hydrophilic properties (contact angle of ∼80), gold-coated nanoporous PFA films exhibit superhydrophobicity (contact angle of ∼150), thus resulting in a large-area flexible superhydrophobic plasmonic platform.

20. "Plasmonic-photonic interference coupling in submicrometer amorphous TiO2-Ag nanoarchitectures", R. S. Hyam, J. Jeon, S. Chae, Y. T. Park, S. J. Kim, B. Lee, C. Lee, D. Choi, Langmuir 33, 12398 (2017) (Link) 

In this study, we report the crystallinity effects of submicrometer titanium dioxide (TiO2) nanotube (TNT) incorporated with silver (Ag) nanoparticles (NPs) on surfaceenhanced Raman scattering (SERS) sensitivity. Furthermore, we demonstrate the SERS behaviors dependent on the plasmonic−photonic interference coupling (P-PIC) in the TNT-AgNP nanoarchitectures. Amorphous TNTs (A-TNTs) are synthesized through a two-step anodization on titanium (Ti) substrate, and crystalline TNTs (C-TNTs) are then prepared by using thermal annealing process at 500 °C in air. After thermally evaporating 20 nm thick Ag on TNTs, we investigate SERS signals according to the crystallinity and P-PIC on our TNT-AgNP nanostructures. (A-TNTs)-AgNP substrates show dramatically enhanced SERS performance as compared to (C-TNTs)-AgNP substrates. We attribute the high enhancement on (A-TNTs)-AgNP substrates with electron confinement at the interface between A-TNTs and AgNPs as due to the high interfacial barrier resistance caused by band edge positions. Moreover, the TNT length variation in (A-TNTs)-AgNP nanostructures results in different constructive or destructive interference patterns, which in turn affects the P-PIC. Finally, we could understand the significant dependency of SERS intensity on P-PIC in (A-TNTs)-AgNP nanostructures. Our results thus might provide a suitable design for a myriad of applications of enhanced EM on plasmonic-integrated devices.

19. "Enhanced Heat Transfer using Metal Foam Liquid Supply Layers for Micro Heat Spreaders", S. Ryu, J. Han, J. Kim, C. Lee*, Y. Nam*, International Journal of Heat and Mass Transfer 108, 2338 (2017) (Link) 

We propose a nanostructured metal foam liquid supply layer that can efficiently provide operating fluid to evaporator hot spots and can be easily integrated within micro heat spreaders. The liquid supply layer is incorporated onto the micropost evaporator wicks to enhance the capillary performance by combining the high permeability of liquid supply layers and the high capillary pressure of micropost wicks. The coverage ratio (CR) between the liquid supply layer and the evaporator wicks was varied from 15% to 100% to find the proper CR for efficiently increasing the liquid supply performance with minimizing the parasitic thermal resistance. By incorporating the liquid supply layer of CR 33% onto the Cu micropost wicks of 0.4 solid fraction, the results show that a high (>6 W/cm2 K) and stable heat transfer coefficient can be achieve at a high heat flux range (>400 W/cm2), which outweighs the performance of previously reported evaporator wicks. The achieved maximum heat flux was over 150% higher than the same wicks without the liquid supply layer. Our work shows the importance of the efficient liquid supply to hot spots and provides the strategy to increase the heat transfer performance at high heat flux region. The suggested liquid supply layer will help develop micro heat spreaders for the thermal management of high power density microprocessors, IGBTs and thermophotovoltaic cells.

18. "Nanoscale Dynamics versus Surface Interactions: What Dictates Osmotic Transport?", C. Lee, C. Cottin-Bizonne, R. Fulcrand, L. Joly, C. Ybert, Journal of Physical Chemistry Letters 8, 478 (2017) (Link) 

The classical paradigm for osmotic transport has long related the induced flow direction to the solute membrane interactions, with the low-to-high concentration flow a direct consequence of the solute rejection from the semipermeable membrane. In principle, the same was thought to occur for the newly demonstrated membrane-free osmotic transport named diffusio-osmosis. Using a recently proposed nanofluidic setup, we revisit this cornerstone of osmotic transport by studying the diffusio-osmotic flows generated at silica surfaces by either poly(ethylene)glycol polymers or ethanol molecules in aqueous solutions. Strikingly, both neutral solutes yield osmotic flows in the usual low to high concentration direction, in contradiction with their propensity to adsorb on silica. Considering theoretically and numerically the intricate nature of the osmotic response that combines molecular-scale surface interaction and near-wall dynamics, these findings are rationalized within a generalized framework. These elements constitute a step forward toward a finer understanding of osmotically driven flows, at the core of rapidly growing fields ranging from energy harvesting to active matter.

17. "Water penetration through a superhydrophobic mesh during a drop impact", S. Ryu, P. Sen, Y. Nam*, C. Lee*, Physical Review Letters 118, 014501 (2017) (Link) 

When a water drop impacts a mesh having submillimeter pores, a part of the drop penetrates through the mesh if the impact velocity is sufficiently large. Here we show that different surface wettability, i.e., hydrophobicity and superhydrophobicity, leads to different water penetration dynamics on a mesh during drop impact.We show, despite the water repellence of a superhydrophobic surface, that water can penetrate a superhydrophobic mesh more easily (i.e., at a lower impact velocity) over a hydrophobic mesh via a penetration mechanism unique to a superhydrophobic mesh. On a superhydrophobic mesh, the water penetration can occur during the drop recoil stage, which appears at a lower impact velocity than the critical impact velocity for water penetration right upon impact.We propose that this unique water penetration on a superhydrophobic mesh can be attributed to the combination of the hydrodynamic focusing and the momentum transfer from the water drop when it is about to bounce off the surface, at which point the water drop retrieves most of its kinetic energy due to the negligible friction on superhydrophobic surfaces.

16. "Superhydrophobic drag reduction in laminar flows: Critical review", C. Lee*, C.-H. Choi, C.-J. Kim, Experiments in Fluids 57(12), 176 (2016) (Link) 

A gas in between micro- or nanostructures on a submerged superhydrophobic (SHPo) surface allows the liquid on the structures to flow with an effective slip. If large enough, this slippage may entail a drag reduction appreciable for many flow systems. However, the large discrepancies among the slippage levels reported in the literature have led to a widespread misunderstanding on the drag-reducing ability of SHPo surfaces. Today we know that the amount of slip, generally quantified with a slip length, is mainly determined by the structural features of SHPo surfaces, such as the pitch, solid fraction, and pattern type, and further affected by secondary factors, such as the state of the liquid–gas interface. Reviewing the experimental data of laminar flows in the literature comprehensively and comparing them with the theoretical predictions, we provide a global picture of the liquid slip on structured surfaces to assist in rational design of SHPo surfaces for drag reduction. Because the trapped gas, called plastron, vanishes along with its slippage effect in most application conditions, lastly we discuss the recent efforts to prevent its loss. This review is limited to laminar flows, for which the SHPo drag reduction is reasonably well understood.

15. "The effects of surface wettability on the fog and dew moisture harvesting performance on tubular surfaces", D. Seo, J. Lee, C. Lee*, Y. Nam*, Scientific Reports 6, 24276 (2016) (Link) 

The efficient water harvesting from air-laden moisture has been a subject of great interest to address world-wide water shortage issues. Recently, it has been shown that tailoring surface wettability can enhance the moisture harvesting performance. However, depending on the harvesting condition, a different conclusion has often been reported and it remains unclear what type of surface wettability would be desirable for the efficient water harvesting under the given condition. Here we compare the water harvesting performance of the surfaces with various wettability under two different harvesting conditions–dewing and fogging, and show that the different harvesting efficiency of each surface under these two conditions can be understood by considering the relative importance of the water capturing and removal efficiency of the surface. At fogging, the moisture harvesting performance is determined by the water removal efficiency of the surface with the oil-infused surfaces exhibiting the best performance. Meanwhile, at dewing, both the water capturing and removal efficiency are crucial to the harvesting performance. And well-wetting surfaces with a lower barrier to nucleation of condensates exhibit a better harvesting performance due to the increasing importance of the water capture efficiency over the water removal efficiency at dewing.

14. "Two types of Cassie-to-Wenzel wetting transitions on superhydrophobic surfaces during drop impact", C. Lee*, Y. Nam*, H. Lastakowski, J. I. Hur, S. Shin, A.-L. Biance, C. Pirat, C.-J. Kim, C. Ybert, Soft Matter 11, 4592 (2015) (2015 Soft Matter HOT paper) (Link) 

Despite the fact that superhydrophobic surfaces possess useful and unique properties, their practical application has remained limited by durability issues. Among those, the wetting transition, whereby a surface gets impregnated by the liquid and permanently loses its superhydrophobicity, certainly constitutes the most limiting aspect under many realistic conditions. In this study, we revisit this so-called Cassie-to-Wenzel transition (CWT) under the broadly encountered situation of liquid drop impact. Using model hydrophobic micropillar surfaces of various geometrical characteristics and high speed imaging, we identify that CWT can occur through different mechanisms, and at different impact stages. At early impact stages, right after contact, CWT occurs through the well established dynamic pressure scenario of which we provide here a fully quantitative description. Comparing the critical wetting pressure of surfaces and the theoretical pressure distribution inside the liquid drop, we provide not only the CWT threshold but also the hardly reported wetted area which directly affects the surface spoiling. At a later stage, we report for the first time to our knowledge, a new CWT which occurs during the drop recoil toward bouncing. With the help of numerical simulations, we discuss the mechanism underlying this new transition and provide a simple model based on impulse conservation which successfully captures the transition threshold. By shedding light on the complex interaction between impacting water drops and surface structures, the present study will facilitate designing superhydrophobic surfaces with a desirable wetting state during drop impact.

13. "Near-wall nanovelocimetry based on total internal reflection fluorescence with continuous tracking", Z. Li, L. D'eramo, C. Lee, F. Monti, M. Yonger, P. Tabeling, B. Chollet, B. Bresson, Y. Tran, Journal of Fluid Mechanics 766, 147 (2015) (Link) 

The goal of this work is to make progress in the domain of near-wall velocimetry. The technique we use is based on the tracking of nanoparticles in an evanescent field, close to a wall, a technique called TIRF (total internal reflection fluorescence)-based velocimetry. The particles are filmed continuously, with no time gap between two frames, so that no information on their trajectories is lost. A number of biases affect the measurements: Brownian motion, heterogeneities induced by the walls, statistical biases, photobleaching, polydispersivity and limited depth of field. Their impacts are quantified by carrying out Langevin stochastic simulations, in a way similar to Guasto & Breuer (Exp. Fluids, vol. 47, 2009, pp. 1059–1066). By using parameters calibrated separately or known, we obtain satisfactory agreement between experiments and simulations, concerning the intensity density distributions, velocity fluctuation distributions and the slopes of the linear velocity profiles. Slip lengths measurements, taken as benchmarks for analysing the performances of the technique, are carried out by extrapolating the corrected velocity profiles down to the origin along with determining the wall position with an unprecedented accuracy. For hydrophilic surfaces, we obtain 15 nm for the slip length in sucrose solutions and 910 nm in water, and for hydrophobic surfaces, 32 +/-5 nm for sucrose solutions and 55 +/-9 nm for water. The errors (based on 95% confidence intervals) are significantly smaller than the state of the art, but more importantly, the method demonstrates for the first time a capacity to measure slippage with a satisfactory accuracy, while providing a local information on the flow structure with a nanometric spatial precision and velocity errors of a few per cent. Our study confirms the discrepancy already pointed out in the literature between numerical and experimental slip length estimates. With the progress conveyed by the present work, TIRF-based technique with continuous tracking can be considered as a quantitative method for investigating flow properties close to walls, providing both global and local information on the flow.

12. "Droplet Coalescence on Water Repellant Surfaces", Y. Nam, D, Seo, C. Lee, S. Shin, Soft Matter 11, 154 (2015) (Link) 

We report our hydrodynamic and energy analyses of droplet coalescence on water repellent surfaces including hydrophobic, superhydrophobic and oil-infused superhydrophobic surfaces. The receding contact angle has significant effects on the contact line dynamics since the contact line dissipation was more significant during the receding mode than advancing. The contact line dynamics is modeled by the damped harmonic oscillation equation, which shows that the damping ratio and angular frequency of merged droplets decrease as the receding contact angle increases. The fast contact line relaxation and the resulting decrease in base area during coalescence were crucial to enhance the mobility of coalescing sessile droplets by releasing more surface energy with reducing dissipation loss. The superhydrophobic surface converts 42% of the released surface energy to the kinetic energy via coalescence before the merged droplet jumps away from the surface, while oil-infused superhydrophobic and hydrophobic surfaces convert 30% and 22%, respectively, for the corresponding time. This work clarifies the mechanisms of the contact line relaxation and energy conversion during the droplet coalescence on water repellent surfaces, and helps develop water repellent condensers.

11. "Influence of Geometric Patterns of Microstructured Superhydrophobic Surfaces on Water Harvesting Performance via Dewing", D. Seo, C. Lee*, Y. Nam*, Langmuir 30, 15468 (2014) (Link) 

On superhydrophobic (SHPo) surfaces, either of two wetting states the Cassie state (i.e., nonwetted state) and the Wenzel state (i.e., wetted state)can be observed depending on the thermodynamic energy of each state and external conditions. Each wetting state leads to quite a distinctive dynamic characteristic of the water drop on SHPo surfaces, and it has been of primary interest to understand or induce the desirable wetting state for relevant thermofluid engineering applications. In this study, we investigate how the wetting state of microstructured SHPo surfaces influences the water-harvesting performance via dewing by testing two different patterns, including posts and grates with varying structural parameters. On grates, the observed Cassie wetting state during condensation is well described by the thermodynamic energy criteria, and small condensates can be efficiently detached from the surfaces because of the small contact line pinning force of Cassie droplets. Meanwhile, on posts, the observed wetting state is dominantly the Wenzel state regardless of the thermodynamic energy of each state, and the condensates are shed only after they grow to a sufficiently large size to overcome the much larger pinning force of the Wenzel state. On the basis of the mechanical force balance model and energy barrier consideration, we attribute the difference in the droplet shedding characteristics to the different dynamic pathway from the Wenzel state to the Cassie state between posts and grates. Overall, the faster droplet shedding helps to enhance the water-harvesting performance of the SHPo surfaces by facilitating condensation on the droplet-free area, as evidenced by the best water-harvesting performance of grates on the Cassie state among the tested surfaces.

10. "Drop Impact Dynamics on Oil-Infused Nanostructured Surfaces", C. Lee*, H. Kim, Y. Nam*, Langmuir 30, 8400-8407 (2014) (Link) 

We experimentally investigated the impact dynamics of a water drop on oil-infused nanostructured surfaces using high-speed microscopy and scalable metal oxide nano surfaces. The effects of physical properties of the oil and impact velocity on complex fluid dynamics during drop impact were investigated. We show that the oil viscosity does not have significant effects on the maximal spreading radius of the water drop, while it moderately affects the retraction dynamics. The oil viscosity also determines the stability of the infused lubricant oil during the drop impact; i.e., the low viscosity oil layer is easily displaced by the impacting drop, which is manifested by a residual mark on the impact region and earlier initiation of prompt splashing. Also, because of the liquid (water)-liquid (oil) interaction on oil-infused surfaces, various instabilities are developed at the rim during impact under certain conditions, resulting in the flower-like pattern during retraction or elongated filaments during spreading. We believe that our findings will contribute to the rational design of oil-infused surfaces under drop impact conditions by illuminating the complex fluid phenomena on oil-infused surfaces during drop impact.

9. "Osmotic flow through fully permeable nanochannels", C. Lee, C. Cottin-Bizonne, A.-L. Biance, P. Joseph, L. Bocquet, C. Ybert, Physical Review Letters 112, 244501 (2014) (Editor's suggestion) (Link) 

Osmosis across membranes is intrinsically associated with the concept of semipermeability. Here, however, we demonstrate that osmotic flow can be generated by solute gradients across nonselective, fully permeable nanochannels. Using a fluorescence imaging technique, we are able to measure the water flow rate inside single nanochannels to an unprecedented sensitivity of femtoliters per minute flow rates. Our results indicate the onset of a convective liquid motion under salinity gradients, from the higher to lower electrolyte concentration, which is attributed to diffusio-osmotic transport. To our knowledge, this is the first experimental evidence and quantitative investigation of this subtle interfacially driven transport, which need to be accounted for in nanoscale dynamics. Finally, diffusio-osmotic transport under a neutral polymer gradient is also demonstrated. The experiments highlight the entropic depletion of polymers that occurs at the nanochannel surface, resulting in convective flow in the opposite direction to that seen for electrolytes.

8. "Dynamical role of slip heterogeneities in confined flows", A.-L. Vayssade, C. Lee, E. Terriac, F. Monti, M. Cloitre, P. Tabeling, Physical Review E 89, 052309 (2014) (Link) 

We demonstrate that flows in confined systems are controlled by slip heterogeneities below a certain size. To showthis we image the motion of soft glassy suspensions in microchannels whose innerwalls impose different slip velocities. As the channel height decreases, the flow ceases to have the symmetric shape expected for yield-stress fluids. A theoretical model accounts for the role of slip heterogeneities and captures the velocity profiles. We generalize these results by introducing a length scale, valid for all fluids, below which slip heterogeneities dominate the flow in confined systems. General implications of this notion, concerning the interplay between slip and confinement, are presented.

7. "Large apparent electric size of solid-state nanopore due to spatially extended surface conduction", C. Lee, L. Joly, A. Sira, A.-L. Biance, R. Fulcrand, L. Bocquet, Nano Letters 12, 4037 (2012) (Link) 

Ion transport through nanopores drilled in thin membranes is central to numerous applications, including biosensing and ion selective membranes. This paper reports experiments, numerical calculations, and theoretical predictions demonstrating an unexpectedly large ionic conduction in solid-state nanopores, taking its origin in anomalous entrance effects. In contrast to naive expectations based on analogies with electric circuits, the surface conductance inside the nanopore is shown to perturb the three-dimensional electric current streamlines far outside the nanopore in order to meet charge conservation at the pore entrance. This unexpected contribution to the ionic conductance can be interpreted in terms of an apparent electric size of the solid-state nanopore, which is much larger than its geometric counterpart whenever the number of charges carried by the nanopore surface exceeds its bulk counterpart. This apparent electric size, which can reach hundreds of nanometers, can have a major impact on the electrical detection of translocation events through nanopores, as well as for ionic transport in biological nanopores.

6. "Wetting and Active Dewetting on Hierarchically Structured Superhydrophobic Surfaces Immersed in Water", C. Lee, C.-J. Kim, IEEE/ASME Journal of Microelectromechanical Systems 21(3), 712 (2012) (Link) 

When a superhydrophobic (SHPo) surface is fully submerged underwater, the surface becomes wet eventually and cannot recover the SHPo state naturally. In this paper, we fabricate hydrophobic microposts on a hydrophobic nanostructured substrate, i.e., hierarchically structured SHPo surfaces, and investigate their wetting and dewetting processes while immersed in water. All the micropost surfaces get wet, with the conditions of the water only affecting the speed of wetting. All the nanopost surfaces get wet over time after all the microposts on them are wetted. We demonstrate various active means (saturating air in water, bridging the surface to the outside air, and gas generation on the substrate) that can dewet the already wetted microposts as far as the substrate nanoposts remained nonwet. In particular, the SHPo surfaces with the recently developed electrolytic recovery mechanism are shown to maintain an SHPo state even under previously irreconcilable conditions, such as surface defects and high liquid pressure, indefinitely.

5. "Viscoelastic Properties of Bovine Orbital Connective Tissue and Fat: Constitutive Model",L. Yoo, V. Gupta, C. Lee, P. Kavehpore, J. L. Demer, Biomechanics and Modeling in Mechanobiology 10(6), 901 (2011) (Link) 

Reported mechanical properties of orbital connective tissue and fat have been too sparse to model strain-stress relationships underlying biomechanical interactions in strabismus. We performed rheological tests to develop a multi-mode upper convected Maxwell (UCM) model of these tissues under shear loading. From 20 fresh bovine orbits, 30 samples of connective tissue were taken from rectus pulley regions and 30 samples of fatty tissues from the posterior orbit. Additional samples were defatted to determine connective tissue weight proportion, which was verified histologically. Mechanical testing in shear employed a triborheometer to perform: strain sweeps at 0.5-2.0 Hz; shear stress relaxation with 1% strain; viscometry at 0.01-0.5 s(-1) strain rate; and shear oscillation at 1% strain. Average connective tissue weight proportion was 98% for predominantly connective tissue and 76% for fatty tissue. Connective tissue specimens reached a long-term relaxation modulus of 668 Pa after 1,500 s, while corresponding values for fatty tissue specimens were 290 Pa and 1,100 s. Shear stress magnitude for connective tissue exceeded that of fatty tissue by five-fold. Based on these data, we developed a multi-mode UCM model with variable viscosities and time constants, and a damped hyperelastic response that accurately described measured properties of both connective and fatty tissues. Model parameters differed significantly between the two tissues. Viscoelastic properties of predominantly connective orbital tissues under shear loading differ markedly from properties of orbital fat, but both are accurately reflected using UCM models. These viscoelastic models will facilitate realistic global modeling of EOM behavior in binocular alignment and strabismus.

4. "Influence of Surface Hierarchy of Superhydrophobic Surfaces on Liquid Slip", C. Lee, C.-J. Kim, Langmuir 27(7), 4243 (2011)  (Link) 

We investigated how the surface hierarchy of superhydrophobic (SHPo) surfaces influences liquid slip by testing well-defined microposts that have nanoposts only on their top. Contrary to the commonly held belief, our results show that such hierarchical surfaces do not always lead to an increase of slip length despite their reduced solid fraction and enhanced hydrophobicity compared to single-scale surfaces. Adding nanoposts on top of the microposts resulted in an increase of slip length only if the original microposts had a solid fraction above a threshold value. For solid fractions below this threshold, adding nanoposts decreased the slip length. We propose that there were not enough nanoposts on the top surface of very thin microposts to support the liquid pressure, allowing the liquid to intrude down to the top corners of the microposts.

3. "Underwater Restoration and Retention of Gases on Superhydrophobic Surfaces for Drag Reduction", C. Lee, C.-J. Kim, Physical Review Letters 106, 014502 (2011) (Research highlight in Nature)  (Link) 

Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surface structures can hold a lubricating gas film between the solid surface and the liquid in contact with it. However, the types of SHPo surfaces that would produce any meaningful amount of reduction get wet under liquid pressure or at surface defects, both of which are unavoidable in the real world. In this Letter, we solve the above problem by (1) discovering surface structures that allow the restoration of a gas blanket from a wetted state while fully immersed underwater and (2) devising a self-controlled gas-generation mechanism that maintains the SHPo condition under high liquid pressures (tested up to 7 atm) as well as in the presence of surface defects, thus removing a fundamental barrier against the implementation of SHPo surfaces for drag reduction.

2. "Maximizing the Giant Liquid Slip on Superhydrophobic Microstructures by Nanostructuring Their Sidewalls", C. Lee, C.-J. Kim, Langmuir 25(21), 12812 (2009)  (Link) 

In an effort to maximize the liquid slip on superhydrophobic surfaces, we investigate the role of the nanoscale roughness on microscale structures by developing well-defined micro-nano hierarchical structures. The nonwetting stability and slip length on the dual-scale micro-nano structures are measured and compared with those on single-scale micro-smooth structures. A force balance between a liquid pressure and a surface tension indicates that hydrophobic nanostructures on the sidewall of microposts or microgrates would expand the range of the nonwetted state. When a higher gas fraction or a larger pitch can be tested without wetting, a larger slip length is expected on the microstructures. An ideal dual-scale structure is described that isolates the role of the nanostructures, and a fabrication technique is developed to achieve such a micro structure-smooth tops and nanostructured sidewalls. The tests confirm such micronano structures allow a nonwetted state at a higher gas fraction or a larger pitch than the previous micro-smooth structures. As a result, we achieve the maximum slip length of ∼400 μm on the dual-scale structures, an increase of ∼100% over the previous maximum reported on the single-scale (i.e., micro-smooth) structures. The study ameliorates our understanding of the role of each scale on hierarchical structures for a wetting transition and a liquid slip. The resulting giant slip is large enough to influence many fluidic applications, even in macroscale.

1.  "Structured surfaces for a giant liquid slip", C. Lee, C.-H. Choi, C.-J. Kim, Physical Review Letters 101, 064501 (2008) (Research Highlight in Nature)  (Link) 

We study experimentally how two key geometric parameters (pitch and gas fraction) of textured hydrophobic surfaces affect liquid slip. The two are independently controlled on precisely fabricated microstructures of posts and grates, and the slip length of water on each sample is measured using a rheometer system. The slip length increases linearly with the pitch but dramatically with the gas fraction above 90%, the latter trend being more pronounced on posts than on grates. Once the surfaces are designed for very large slips (>20 μm), however, further increase is not obtained in regular practice because the meniscus loses its stability. By developing near-perfect samples that delay the transition from a dewetted (Cassie) to a wetted (Wenzel) state until near the theoretical limit, we achieve giant slip lengths, as large as 185 μm.