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.

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. 2013;8632 :863201.Abstract

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. 2013;38 :366-371.Abstract

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.

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