Kyoo-Chul Park

Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.

Designing multifunctional surfaces that have user-specified interactions with impacting liquids and with incident light is a topic of both fundamental and practical significance. Taking cues from nature, we use tapered conical nanotextures to fabricate the multifunctional surfaces; the slender conical features result in large topographic roughness, while the axial gradient in the effective refractive index minimizes reflection through adiabatic index-matching between air and the substrate. Precise geometric control of the conical shape and slenderness of the features as well as periodicity at the nanoscale are all keys to optimizing the multifunctionality of the textured surface, but at the same time, these demands pose the toughest fabrication challenges. Here we report a systematic approach to concurrent design of optimal structures in the fluidic and optical domains and a fabrication procedure that achieves the desired aspect ratios and periodicities with few defects and large pattern area. Our fabricated nanostructures demonstrate structural superhydrophilicity or, in combination with a suitable chemical coating, robust superhydrophobicity. Enhanced polarization-independent optical transmission exceeding 98% has also been achieved over a broad range of bandwidth and incident angles. These nanotextured surfaces are also robustly antifogging or self-cleaning, offering potential benefits for applications such as photovoltaic solar cells.

Condensation on slippery asymmetric bumps

Controlling dropwise condensation is fundamental to water-harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches based on microscale, nanoscale or molecular-scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach—based on principles derived from Namib desert beetles, cacti, and pitcher plants—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and using theoretical modelling, we show how to maximize vapour diffusion fluxat the apex of convex millimetric bumps by optimizing the radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope, analogous to cactus spines, directly couples facilitated droplet growth with fast directional transport, by creating a free-energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher-plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity and overcome the effect of an unfavourable temperature gradient. We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces. We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications.

Scale dependence of omniphobic mesh surfaces

We provide a simple design chart framework to predict the apparent contact angle on a textured surface in terms of the equilibrium contact angle on a chemically identical smooth surface and details of the surface topography. For low surface tension liquids such as methanol (gamma(lv) = 22.7 mN/m) and octane (gamma(lv) = 21.6 mN/m), a solid-liquid-air composite interface on a textured surface is inherently metastable. Thus, on application of a sufficient pressure difference (e.g., an externally applied pressure or a sufficiently large Laplace pressure at small droplet size) the metastable composite interface transitions to a fully wetted interface. A dimensionless robustness factor is used to quantify the breakthrough pressure difference necessary to disrupt a metastable composite interface and to predict a priori the existence of a robust composite interface. The impact of the length scale (radius of the cylindrical features R varying from 18 to 114 microm) and the feature spacing ratio (D(*) = (R + D)/R varying from 2.2 to 5.1, where 2D is the spacing between the cylindrical features) on the robustness is illustrated by performing contact angle measurements on a set of dip-coated wire-mesh surfaces, which provide systematically quantifiable cylindrical texture. The design chart for a given feature size R shows how the two independent design parameters--surface chemistry as revealed in the equilibrium contact angle and texture spacing embodied in the dimensionless spacing ratio (D(*))--can be used to develop surfaces with desirably large values of the apparent contact angle and robustness of the metastable composite interface. Most revealing is the scaling of the robustness with the dimensionless parameter l(cap)/R (where l(cap = (gamma(lv)/rho g)(1/2) is the capillary length), which indicates clearly why, in the consideration of self-similar surfaces, smaller is better for producing omniphobic surfaces that resist wetting by liquids with low surface tension.

Optimal Design of Permeable Fiber Network Structures for Fog Harvesting

Fog represents a large untapped source of potable water, especially in arid climates. Numerous plants and animals use textural and chemical features on their surfaces to harvest this precious resource. In this work, we investigate the influence of the surface wettability characteristics, length scale, and weave density on the fog-harvesting capability of woven meshes. We develop a combined hydrodynamic and surface wettability model to predict the overall fog-collection efficiency of the meshes and cast the findings in the form of a design chart. Two limiting surface wettability constraints govern the re-entrainment of collected droplets and clogging of mesh openings. Appropriate tuning of the wetting characteristics of the surfaces, reducing the wire radii, and optimizing the wire spacing all lead to more efficient fog collection. We use a family of coated meshes with a directed stream of fog droplets to simulate a natural foggy environment and demonstrate a five-fold enhancement in the fog-collecting efficiency of a conventional polyolefin mesh. The design rules developed in this work can be applied to select a mesh surface with optimal topography and wetting characteristics to harvest enhanced water fluxes over a wide range of natural convected fog environments.

Multifunctional Inverted Nanocone Arrays for Non‐Wetting, Self‐Cleaning Transparent Surface with High Mechanical Robustness

A multifunctional surface that enables control of wetting, optical reflectivity and mechanical damage of nanostructured interfaces is presented. Our approach is based on imprinting a periodic array of nanosized cones into a UV-curable polyurethane acrylate (PUA), resulting in a self-reinforcing egg-crate topography evenly distributed over large areas up to several cm(2) in size. The resulting surfaces can be either superhydrophilic or superhydrophobic (through subsequent application of an appropriate chemical coating), they minimize optical reflection losses over a broad range of wavelengths and a wide range of angles of incidence, and they also have enhanced mechanical resilience due to greatly improved redistribution of the normal and shearing mechanical loads. The transmissivity and wetting characteristics of the nanoscale egg-crate structure, as well as its resistance to mechanical deformation are analyzed theoretically. Experiments show that the optical performance together with self-cleaning or anti-fogging behavior of the inverted nanocone topography is comparable to earlier designs that have used periodic arrays of nanocones to control reflection and wetting. However the egg-crate structures are far superior in terms of mechanical robustness, and the ability to replicate this topography through several generations is promising for large-scale commercial applications where multifunctionality is important.

Quantification of feather structure, wettability and resistance to liquid penetration.

Birds in the cormorant (Phalacrocoracidae) family dive tens of metres into water to prey on fish while entraining a thin layer of air (a plastron film) within the microstructures of their feathers. In addition, many species within the family spread their wings for long periods of time upon emerging from water. To investigate whether wetting and wing-spreading are related to feather structure, microscopy and photographic studies have previously been used to extract structural parameters for barbs and barbules. In this work, we describe a systematic methodology to characterize the quasi-hierarchical topography of bird feathers that is based on contact angle measurements using a set of polar and non-polar probing liquids. Contact angle measurements on dip-coated feathers of six aquatic bird species (including three from the Phalacrocoracidae family) are used to extract two distinguishing structural parameters, a dimensionless spacing ratio of the barbule (D*) and a characteristic length scale corresponding to the spacing of defect sites. The dimensionless spacing parameter can be used in conjunction with a model for the surface topography to enable us to predict a priori the apparent contact angles of water droplets on feathers as well as the water breakthrough pressure required for the disruption of the plastron on the feather barbules. The predicted values of breakthrough depths in water (1–4 m) are towards the lower end of typical diving depths for the aquatic bird species examined here, and therefore a representative feather is expected to be fully wetted in a typical deep dive. However, thermodynamic surface energy analysis based on a simple one-dimensional cylindrical model of the feathers using parameters extracted from the goniometric analysis reveals that for water droplets on feathers of all six species under consideration, the non-wetting ‘Cassie–Baxter’ composite state represents the global energy minimum of the system. By contrast, for other wetting liquids, such as alkanes and common oils, the global energy minimum corresponds to a fully wetted or Wenzel state. For diving birds, individual feathers therefore spontaneously dewet once the bird emerges out of water, and the ‘wing-spreading’ posture might assist in overcoming kinetic barriers associated with pinning of liquid droplets that retard the rate of drying of the wet plumage of diving birds.

Drop impact on inclined superhydrophobic surfaces

This paper discusses the dynamic behavior of water drops impacting on inclined superhydrophobic surfaces. For a normal impact on a smooth hydrophobic surface, the spreading (or expansion) and retraction dynamics of an impacting drop varies from complete rebound to splashing depending on its Weber number, (We(d)), calculated using the impact speed and diameter d of the drop. For a slanted impact, on the other hand, the impact dynamics depends on two distinct Weber numbers, based on the velocity components normal, (We(nd)), and tangential, (We(td)), to the surface. Impact on superhydrophobic surfaces is even more complicated as the surfaces are covered with micro- to nano-scale texture. Therefore, we develop an expression for an additional set of two Weber numbers, (We(na), We(ta)), which are counterparts to the first set but use the gap distance a between asperities on the textured surface as the characteristic length. We correlate the derived Weber numbers with the impact dynamics on tilted surfaces covered with three different types of texture: (i) posts, (ii) ridges aligned with and (iii) ridges perpendicular to the impact direction. Results suggest that the first two Weber numbers, (We(nd), We(td)), affect the impact dynamics of a drop such as the degree of drop deformation as long as the superhydrophobicity remains intact. On the other hand, the Weber number We(na) determines the transition from the superhydrophobic Cassie-Baxter regime to the fully-wetted Wenzel regime. Accuracy of our model becomes lower at a high tilting angle (75°), due to the change in the transition mechanism.

Dropwise condensation on hydrophobic bumps and dimples

Surface topography plays an important role in promoting or suppressing localized condensation. In this work, we study the growth of water droplets on hydrophobic convex surface textures such as bumps and concave surface textures such as dimples with a millimeter scale radius of curvature. We analyze the spatio-temporal droplet size distribution under a supersaturation condition created by keeping the uniform surface temperature below the dew point and show its relationship with the sign and magnitude of the surface curvature. In particular, in contrast to the well-known capillary condensation effect, we report an unexpectedly less favorable condensation on smaller, millimeter-scale dimples where the capillary condensation effect is negligible. To explain these experimental results, we numerically calculated the diffusion flux of water vapor around the surface textures, showing that its magnitude is higher on bumps and lower on dimples compared to a flat surface. We envision that our understanding of milli...

Superoleophilic Titania Nanoparticle Coatings with Fast Fingerprint Decomposition and High Transparency

Low surface tension sebaceous liquids such as human fingerprint oils are readily deposited on high energy surfaces such as clean glass, leaving smudges that significantly lower transparency. There have been several attempts to prevent formation of these dactylograms on glass by employing oil-repellent textured surfaces. However, nanotextured superoleophobic coatings typically scatter visible light, and the intrinsic thermodynamic metastability of the composite superoleophobic state can result in failure of the oil repellency under moderate contact pressure. We develop titania-based porous nanoparticle coatings that are superoleophilic and highly transparent and which exhibit short time scales for decomposition of fingerprint oils under ultraviolet light. The mechanism by which a typical dactylogram is consumed combines wicking of the sebum into the nanoporous titania structure followed by photocatalytic degradation. We envision a wide range of applications because these TiO2 nanostructured surfaces remain photocatalytically active against fingerprint oils in natural sunlight and are also compatible with flexible glass substrates.