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.

Hou X, Hu Y, Grinthal A, Khan M, Aizenberg J. Liquid-based gating mechanism with tunable multiphase selectivity and antifouling behaviour. Nature [Internet]. 2015;519 (7541) :70-73. Full TextAbstract
Living organisms make extensive use of micro- and nanometre-sized pores as gatekeepers for controlling the movement of fluids, vapours and solids between complex environments. The ability of such pores to coordinate multiphase transport, in a highly selective and subtly triggered fashion and without clogging, has inspired interest in synthetic gated pores for applications ranging from fluid processing to 3D printing and lab-on-chip systems. But although specific gating and transport behaviours have been realized by precisely tailoring pore surface chemistries and pore geometries, a single system capable of controlling complex, selective multiphase transport has remained a distant prospect, and fouling is nearly inevitable. Here we introduce a gating mechanism that uses a capillary-stabilized liquid as a reversible, reconfigurable gate that fills and seals pores in the closed state, and creates a non-fouling, liquid-lined pore in the open state. Theoretical modelling and experiments demonstrate that for each transport substance, the gating threshold—the pressure needed to open the pores—can be rationally tuned over a wide pressure range. This enables us to realize in one system differential response profiles for a variety of liquids and gases, even letting liquids flow through the pore while preventing gas from escaping. These capabilities allow us to dynamically modulate gas–liquid sorting in a microfluidic flow and to separate a three-phase air–water–oil mixture, with the liquid lining ensuring sustained antifouling behaviour. Because the liquid gating strategy enables efficient long-term operation and can be applied to a variety of pore structures and membrane materials, and to micro- as well as macroscale fluid systems, we expect it to prove useful in a wide range of applications.
Mishchenko L, Aizenberg J, Hatton BD. Spatial Control of Condensation and Freezing on Superhydrophobic Surfaces with Hydrophilic Patches. Adv. Funct. Mater. [Internet]. 2013;23 (36) :4577-4584. Publisher's VersionAbstract
Certain natural organisms use micro‐patterned surface chemistry, or ice‐nucleating species, to control water condensation and ice nucleation for survival under extreme conditions. As an analogy to these biological approaches, it is shown that functionalized, hydrophilic polymers and particles deposited on the tips of superhydrophobic posts induce precise topographical control over water condensation and freezing at the micrometer scale. A bottom‐up deposition process is used to take advantage of the limited contact area of a non‐wetting aqueous solution on a superhydrophobic surface. Hydrophilic polymer deposition on the tips of these geometrical structures allows spatial control over the nucleation, growth, and coalescence of micrometer‐scale water droplets. The hydrophilic tips nucleate water droplets with extremely uniform nucleation and growth rates, uniform sizes, an increased stability against coalescence, and asymmetric droplet morphologies. Control of freezing behavior is also demonstrated via deposition of ice‐nucleating AgI nanoparticles on the tips of these structures. This combination of the hydrophilic polymer and AgI particles on the tips was used to achieve templating of ice nucleation at the micrometer scale. Preliminary results indicate that control over ice crystal size, spatial symmetry, and position might be possible with this method. This type of approach can serve as a platform for systematically analyzing micrometer‐scale condensation and freezing phenomena, and as a model for natural systems.
Phillips KR, Vogel N, Burgess IB, Perry CC, Aizenberg J. Directional Wetting in Anisotropic Inverse Opals. Langmuir [Internet]. 2014;30 (25) :7615-7620. Publisher's VersionAbstract

Porous materials display interesting transport phenomena due to restricted motion of fluids within the nano- to microscale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy.

Daniel D, Mankin MN, Belisle RA, Wong T-S, Aizenberg J. Lubricant-infused micro/nano-structured surfaces with tunable dynamic omniphobicity at high temperatures. Appl. Phys. Lett. [Internet]. 2013;102 :231603. Full TextAbstract
Omniphobic surfaces that can repel fluids at temperatures higher than 100 °C are rare. Most state-of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is maintained throughout micro/nanotextures leading to high mobility of liquids. However, such behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases significantly. Here, we demonstrate a class of lubricant-infused structuredsurfaces that can maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at least 200 °C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of 1000.
Yao X, Dunn S, Kim P, Duffy M, Alvarenga J, Aizenberg J. Fluorogel Elastomers with Tunable Transparency, Elasticity, Shape- Memory, and Antifouling Properties. Angew. Chem. Int. Ed [Internet]. 2014;53 (17) :4418-4422. Full TextAbstract

Omniphobic fluorogel elastomers were prepared by photocuring perfluorinated acrylates and a perfluoropolyether crosslinker. By tuning either the chemical composition or the temperature that control the crystallinity of the resulting polymer chains, a broad range of optical and mechanical properties of the fluorogel can be achieved. After infusing with fluorinated lubricants, the fluorogels showed excellent resist- ance to wetting by various liquids and anti-biofouling behavior, while maintaining cytocompatiblity.

Kreder MJ, Alvarenga J, Kim P, Aizenberg J. Design of anti-icing surfaces: smooth, textured or slippery?. Nat. Rev. Mater. 2016;1 (1) :15003.Abstract
Passive anti-icing surfaces, or icephobic surfaces, are an area of great interest because of their significant economic, energy and safety implications in the prevention and easy removal of ice in many facets of society. The complex nature of icephobicity, which requires performance in a broad range of icing scenarios, creates many challenges when designing ice-repellent surfaces. Although superhydrophobic surfaces incorporating micro- or nanoscale roughness have been shown to prevent ice accumulation under certain conditions, the same roughness can be detrimental in other environments. Surfaces that present a smooth liquid interface can eliminate some of the drawbacks of textured superhydrophobic surfaces, but additional study is needed to fully realize their potential. As attention begins to shift towards alternative anti-icing strategies, it is important to consider and to understand the nature of ice repellency in all environments to identify the limitations of current solutions and to design new materials with robust icephobicity.
Yao X, Hu Y, Grinthal A, Wong T-S, Mahadevan L, Aizenberg J. Adaptive fluid-infused porous films with tunable transparency and wettability. Nature Materials [Internet]. 2013;12 :529-534. Full TextAbstract
Materials that adapt dynamically to environmental changes are currently limited to two-state switching of single properties, and only a small number of strategies that may lead to materials with continuously adjustable characteristics have been reported1-3. Here we introduce adaptive surfaces made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film's transparency and its ability to continuously manipulate various low-surface-tension droplets from free-sliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications.
Burgess IB, Aizenberg J, Loncar M. Creating bio-inspired hierarchical 3D–2D photonic stacks via planar lithography on self-assembled inverse opals. Bioinspiration & Biomimetics [Internet]. 2013;8 :045004. Publisher's VersionAbstract
Structural hierarchy and complex 3D architecture are characteristics of biological photonic designs that are challenging to reproduce in synthetic materials. Top–down lithography allows for designer patterning of arbitrary shapes, but is largely restricted to planar 2D structures. Self-assembly techniques facilitate easy fabrication of 3D photonic crystals, but controllable defect-integration is difficult. In this paper we combine the advantages of top–down and bottom–up fabrication, developing two techniques to deposit 2D-lithographically-patterned planar layers on top of or in between inverse-opal 3D photonic crystals and creating hierarchical structures that resemble the architecture of the bright green wing scales of the butterfly, Parides sesostris. These fabrication procedures, combining advantages of both top–down and bottom–up fabrication, may prove useful in the development of omnidirectional coloration elements and 3D–2D photonic crystal devices.