This is especially in situations where the water is in the form of vapor or tiny droplets. Enhancing the mobility of liquid droplets on rough surfaces has applications ranging from condensation heat transfer for heat exchangers in power plants to more efficient water harvesting in arid regions where collecting fog droplets on coated meshes provides drinking water and irrigation for agriculture to the prevention of icing and frosting on aircraft wings.
“This represents a fundamentally new concept in engineered surfaces,” said Assistant Prof Tak-Sing Wong. “Our surfaces combine the unique surface architectures of lotus leaves and pitcher plants, in such a way that these surfaces possess both high surface area and a slippery interface to enhance droplet collection and mobility. Mobility of liquid droplets on rough surfaces is highly dependent on how the liquid wets the surface. We have demonstrated for the first time experimentally that liquid droplets can be highly mobile when in the Wenzel state.”
Liquid droplets on rough surfaces come in one of 2 states, Cassie, in which the liquid partially floats on a layer of air or gas, and Wenzel, in which the droplets are in full contact with the surface, trapping or pinning them. “Through careful, systematic analysis, we found that the Wenzel equation does not apply for highly wetting liquids,” said Birgitt Boschitsch Stogin. “Droplets on conventional rough surfaces are mobile in the Cassie state and pinned in the Wenzel state. The sticky Wenzel state results in many problems in condensation heat transfer, water harvesting and ice removal. Our idea is to solve these problems by enabling Wenzel state droplets to be mobile,” said Xianming Dai.
Method: To make Wenzel state droplets mobile, they etched micrometer scale pillars into a silicon surface using photolithography and deep reactive-ion etching, and then created nanoscale textures on the pillars by wet etching. They then infused the nanotextures with a layer of lubricant that completely coated the nanostructures, resulting in greatly reduced pinning of the droplets. The nanostructures also greatly enhanced lubricant retention compared to the microstructured surface alone.
The same design principle can be easily extended to other materials beyond silicon, such as metals, glass, ceramics and plastics. This will open the search for a new, unified model of wetting physics that explains wetting phenomena on rough surfaces such as theirs.
http://news.psu.edu/story/367640/2015/08/31/research/engineered-surface-unsticks-sticky-water-droplets
Schematic showing a new engineered surface that can repel liquids in any state of wetness. Credit: Xianming Dai, Chujun Zeng and Tak-Sing Wong
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