Choongyeop Lee

Maximizing the giant liquid slip on superhydrophobic microstructures by nanostructuring their sidewalls.

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 microstructure-smooth tops and nanostructured sidewalls. The tests confirm such micro-nano 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 approximately 400 microm on the dual-scale structures, an increase of approximately 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.

Influence of surface hierarchy of superhydrophobic surfaces on liquid slip.

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.

Large apparent electric size of solid-state nanopores due to spatially extended surface conduction.

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.

The effects of surface wettability on the fog and dew moisture harvesting performance on tubular surfaces

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.

Drop impact dynamics on oil-infused nanostructured surfaces.

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.

Influence of Geometric Patterns of Microstructured Superhydrophobic Surfaces on Water-Harvesting Performance via Dewing

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.

Wetting and Active Dewetting Processes of Hierarchically Constructed Superhydrophobic Surfaces Fully Immersed in Water

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.

Nanoscale Dynamics versus Surface Interactions: What Dictates Osmotic Transport?

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.

Water Penetration through a Superhydrophobic Mesh During a Drop Impact

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.

Plasmonic-photonic interference coupling in submicrometer amorphous TiO2-Ag nanoarchitectures

In this study, we report the crystallinity effects of submicrometer titanium dioxide (TiO2) nanotube (TNT) incorporated with silver (Ag) nanoparticles (NPs) on surface-enhanced 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.