Wettability

The need to fend off water is as fundamental as the need to acquire it: water absorption by buildings fosters mold growth and structural breakdown, stagnant surface water breeds disease, and waterlogged clothing interferes with body temperature regulation. 

We traditionally rely on chemical coatings to prevent water absorption and retention, but these wear off over time and can be toxic. In contrast, many organisms use built-in topography: water striders keep their legs dry, mosquitoes defog their eyes, and leaves shed raindrops by limiting water contact to the tips of nanoscale bristles on their surfaces. Air fills the rest of the space under the drop, such that the bristles create a patterned air-solid surface on which macroscopic droplets slide and molecules within each droplet diffuse largely as if the drop were in air. 

We are investigating how patterned features govern motion at these unique interfaces, and have recently optimized liquid-surface dynamics to design ice-preventive materials that deflect impacting droplets at sub-freezing temperatures and nucleate only unstable, low-adhesion ice below that. Since topographic patterns disappear if the bristles lie down, water resistance can be turned on and off simply by bending or tilting, and we use this unique feature to design materials that reversibly switch between hydrophobic and hydrophilic behavior in response to environmental conditions. While liquids other than water are more difficult to resist due to their stronger tendency to spread on a surface, we have recently made the surprising discovery that biofilm – a bacterial commune encased in slime – has a unique multiscale topography that fends off not only water but an unprecedented assortment of other liquids, and we are designing previously elusive resilient, highly nonwetting materials based on our intriguing new role model.

Daniel D, Timonen JVI, Li R, Velling SJ, Kreder MJ, Tetreault A, Aizenberg J. Origins of liquid-repellency on structured, flat, and lubricated surfaces . Phys. Rev. Lett. 2018.Abstract
There are currently three main classes of liquid-repellent surfaces: micro-/nano-structured superhydrophobic surfaces, flat surfaces grafted with `liquid-like' polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measured the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities U = 0.01--1 mm/s using a cantilever force sensor with sub-μN accuracy, and correlated it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal (lessthan 0.1s) resolutions. We find that the dissipative force---due to very different physical mechanisms at the contact line---is independent of velocity on superhydrophobic surfaces, but depends non-linearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.
Yao Y, Aizenberg J, Park K-C. Dropwise Condensation on Hydrophobic Bumps and Dimples. Appl. Phys. Lett. [Internet]. 2018;112 (15) :151605. Full TextAbstract
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 millimetric surface topography can be applied to improve the energy efficiency of condensation in applications such as water harvesting, heating, ventilation, and air conditioning systems for buildings and transportation, heat exchangers, thermal desalination plants, and fuel processing systems.
Daniel D, Yao X, Aizenberg J. Stable Liquid Jets Bouncing off Soft Gels. Phys. Rev. Lett. [Internet]. 2018;120 (2) :028006. Publisher's VersionAbstract

A liquid jet can stably bounce off a sufficiently soft gel by following the contour of the dimple created upon impact. This new phenomenon is insensitive to the wetting properties of the gels and was observed for different liquids over a wide range of surface tensions, γ = 24 − 72 mN/m. In contrast, other jet rebound phenomena are typically sensitive to γ: only a high γ jet rebounds off a hard solid (e.g. superhydrophobic surface) and only a low γ jet bounces off a liquid bath. This is because an air layer must be stabilized between the two interfaces. For a soft gel, no air layer is necessary and the jet rebound remains stable even when there is direct liquid-gel contact.

Daniel D, Timonen JVI, Li R, Velling SJ, Aizenberg J. Oleoplaning droplets on lubricated surfaces. Nat. Phys. [Internet]. 2017;13 (10) :1020-1025. Full TextAbstract

Recently, there has been much interest in using lubricated surfaces to achieve extreme liquid repellency: a foreign droplet immiscible with the underlying lubricant layer was shown to slide o at a small tilt angle <5◦ . This behaviour was hypothesized to arise from a thin lubricant overlayer film sandwiched between the droplet and solid substrate, but this has not been observed experimentally. Here, using thin-film interference, we are able to visualize the intercalated film under both static and dynamic conditions. We further demonstrate that for a moving droplet, the film thickness follows the Landau–Levich–Derjaguin law. The droplet is therefore oleoplaning—akin to tyres hydroplaning on a wet road—with minimal dissipative force and no contact line pinning. The techniques and insights presented in this study will inform future work on the fundamentals of wetting for lubricated surfaces and enable their rational design.

Burgess IB, Nerger BA, Raymond KP, Goulet-Hanssens A, Singleton TA, Kinney MH, Shneidman AV, Koay N, Barrett CJ, Loncar M, et al. Wetting in Color: From photonic fingerprinting of liquids to optical control of liquid percolation. Proc. of SPIE [Internet]. 2013;8632 :863201. Publisher's VersionAbstract

We provide an overview of our recent advances in the manipulation of wetting in inverse-opal photonic crystals. Exploiting photonic crystals with spatially patterned surface chemistry to confine the infiltration of fluids to liquidspecific spatial patterns, we developed a highly selective scheme for colorimetry, where organic liquids are distinguished based on wetting. The high selectivity of wetting, upon-which the sensitivity of the response relies, and the bright iridescent color, which disappears when the pores are filled with liquid, are both a result of the highly symmetric pore structure of our inverse-opal films. The application of horizontally or vertically orientated gradients in the surface chemistry allows a unique response to be tailored to specific liquids. While the generic nature of wetting makes our approach to colorimetry suitable for applications in liquid authentication or identification across a broad range of industries, it also ensures chemical non-specificity. However, we show that chemical specificity can be achieved combinatorially using an array of indicators that each exploits different chemical gradients to cover the same dynamic range of response. Finally, incorporating a photo-responsive polyelectrolyte surface layer into the pores, we are able to dynamically and continuously photo-tune the wetting response, even while the film is immersed in liquid. This in situ optical control of liquid percolation in our photonic-crystal films may also provide an error-free means to tailor indicator response, naturally compensating for batch-to-batch variability in the pore geometry.

Wong T-S, Sun T, Feng L, Aizenberg J. Interfacial materials with special wettability. MRS Bulletin [Internet]. 2013;38 :366-371. Publisher's VersionAbstract

Various life forms in nature display a high level of adaptability to their environments through the use of sophisticated material interfaces. This is exemplifi ed by numerous biological systems, such as the self-cleaning of lotus leaves, the water-walking abilities of water striders and spiders, the ultra-slipperiness of pitcher plants, the directional liquid adhesion of butterfl y wings, and the water collection capabilities of beetles, spider webs, and cacti. The versatile interactions of these natural surfaces with fl uids, or special wettability, are enabled by their unique micro/nanoscale surface structures and intrinsic material properties. Many of these biological designs and principles have inspired new classes of functional interfacial materials, which have remarkable potential to solve some of the engineering challenges for industrial and biomedical applications. In this article, we provide a snapshot of the state of the art of biologically inspired materials with special wettability, and discuss some promising future directions for the field.

Utech S, Bley K, Aizenberg J, Vogel N. Tailoring re-entrant geometry in inverse colloidal monolayers to control surface wettability. J. Mater. Chem. A. 2016;4 (18) :6853-6859.Abstract

Controlling the microscopic wetting state of a liquid in contact with a structured surface is the basis for the design of liquid repellent as well as anti-fogging coatings by preventing or enabling a given liquid to infiltrate the surface structures. Similarly, a liquid can be confined to designated surface areas by locally controlling the wetting state, with applications ranging from liquid transport on a surface to creating tailored microenvironments for cell culture or chemical synthesis. The control of the wetting of a low-surfacetension liquid is substantially more difficult compared to water and requires surface structures with overhanging features, known as re-entrant geometries. Here, we use colloidal self-assembly and templating to create two-dimensional nanopore arrays with tailored re-entrant geometry. These pore arrays, termed inverse monolayers, are prepared by backfilling a sacrificial colloidal monolayer with a silica sol–gel precursor material. Varying the precursor concentration enables us to control the degree to which the colloids are embedded into the silica matrix. Upon calcination, nanopores with different opening angles result. The pore opening angle directly correlates with the re-entrant curvature of the surface nanostructures and can be used to control the macroscopic wetting behavior of a liquid sitting on the surface structures. We characterize the wetting of various liquids by static and dynamic contact angles and find correlation between the experimental results and theoretical predictions of the wetting state based on simple geometric considerations. We demonstrate the creation of omniphobic surface coatings that support Cassie–Baxter wetting states for liquids with low surface tensions, including octane (g ¼ 21.7 mN m1). We further use photolithography to spatially confine such low-surface-tension liquids to desired areas of the substrate with high accuracy.

Wei Z, Schneider TM, Kim J, Kim H-J, Aizenberg J, Mahadevan L. Elastocapillary coalescence of plates and pillars. Proc. R. Soc. A. 2015;471 (2175) :20140593.Abstract

When a fluid-immersed array of supported plates or pillars is dried, evaporation leads to the formation of menisci on the tips of the plates or pillars that bring them together to form complex patterns. Building on prior experimental observations, we use a combination of theory and computation to understand the nature of this instability and its evolution in both the two- and three-dimensional setting of the problem. For the case of plates, we explicitly derive the interaction torques based on the relevant physical parameters associated with pillar deformation, contact-line pinning/depinning and fluid volume changes. A Bloch-wave analysis for our periodic mechanical system captures the window of volumes where the two-plate eigenvalue characterizes the onset of the coalescence instability. We then study the evolution of these binary clusters and their eventual elastic arrest using numerical simulations that account for evaporative dynamics coupled to capillary coalescence. This explains both the formation of hierarchical clusters and the sensitive dependence of the final structures on initial perturbations, as seen in our experiments. We then generalize our analysis to treat the problem of pillar collapse in three dimensions, where the fluid domain is completely connected and the interface is a minimal surface with the uniform mean curvature. Our theory and simulations capture the salient features of experimental observations in a range of different situations and may thus be useful in controlling the ensuing patterns.

Singleton TA, Burgess IB, Nerger BA, Goulet-Hanssens A, Koay N, Barrett CJ, Aizenberg J. Photo-tuning of Highly Selective Wetting in Inverse Opals. Soft Matter [Internet]. 2014;10 (9) :1325-1328. Publisher's VersionAbstract

Crack-free inverse opals exhibit a sharply defined threshold wettability for infiltration that has enabled their use as colourimetric indicators for liquid identification. Here we demonstrate direct and continuous photo-tuning of this wetting threshold in inverse opals whose surfaces are function- alized with a polymer doped with azobenzene chromophores.

Burgess IB, Abedzadeh N, Kay TM, Shneidman AV, Cranshaw DJ, Loncar M, Aizenberg J. Tuning and Freezing Disorder in Photonic Crystals using Percolation Lithography. Scientific Reports [Internet]. 2016;6 (1) :19542. Full TextAbstract
Although common in biological systems, synthetic self-assembly routes to complex 3D photonic structures with tailored degrees of disorder remain elusive. Here we show how liquids can be used to finely control disorder in porous 3D photonic crystals, leading to complex and hierarchical geometries. In these optofluidic crystals, dynamically tunable disorder is superimposed onto the periodic optical structure through partial wetting or evaporation. In both cases, macroscopic symmetry breaking is driven by subtle sub-wavelength variations in the pore geometry. These variations direct site-selective infiltration of liquids through capillary interactions. Incorporating cross-linkable resins into our liquids, we developed methods to freeze in place the filling patterns at arbitrary degrees of partial wetting and intermediate stages of drying. These percolation lithography techniques produced permanent photonic structures with adjustable disorder. By coupling strong changes in optical properties to subtle differences in fluid behavior, optofluidic crystals may also prove useful in rapid analysis of liquids.
Vogel N, Belisle RA, Hatton B, Wong TS, Aizenberg J. Transparency and damage tolerance of patternable omniphobic lubricated surfaces based on inverse colloidal monolayers. Nature Communications [Internet]. 2013;4. Publisher's VersionAbstract
A transparent coating that repels a wide variety of liquids, prevents staining, is capable of self-repair and is robust towards mechanical damage can have a broad technological impact, from solar cell coatings to self-cleaning optical devices. Here we employ colloidal templating to design transparent, nanoporous surface structures. A lubricant can be firmly locked into the structures and, owing to its fluidic nature, forms a defect-free, self-healing interface that eliminates the pinning of a second liquid applied to its surface, leading to efficient liquid repellency, prevention of adsorption of liquid-borne contaminants, and reduction of ice adhesion strength. We further show how this method can be applied to locally pattern the repellent character of the substrate, thus opening opportunities to spatially confine any simple or complex fluids. The coating is highly defect-tolerant due to its interconnected, honeycomb wall structure, and repellency prevails after the application of strong shear forces and mechanical damage. The regularity of the coating allows us to understand and predict the stability or failure of repellency as a function of lubricant layer thickness and defect distribution based on a simple geometric model.
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