The leaves of the lotus flower are sometimes considered to be the gold standard when it comes to the ability to repel water and dirt, but now scientists from Penn State University have invented a new hydrophobic nano-surface that actually one-ups the lotus flower.
No matter how slippery natural surfaces like the lotus flower are, tiny water droplets can still stick to them, the research team explained. Their engineered, micro-textured surface, however, outperforms these naturally inspired coatings when water is in tiny droplets or vapor form. It could actually improve water harvesting in dry regions or keep plane wings from icing up.
“This represents a fundamentally new concept in engineered surfaces,” said Tak-Sing Wong, assistant professor of mechanical engineering at Penn State. “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.”
Demonstrating mobility in Wenzel-state liquids
Liquid droplets on rough surfaces come in one of two states: Cassie, in which liquids partially float on a layer of air or gas, and Wenzel, in which they are in complete contact with the surface. This new material marks this first time that experiments have demonstrated that liquid droplets can be highly mobile in the Wenzel state, Wong added.
As the authors explained in a recent edition of the journal ACS Nano, droplets on conventional rough surfaces are mobile when in the Cassie state and pinned in the stickier Wenzel one. This causes issues in condensation heat transfer, water harvesting, and ice removal, and the research team set out to solve those problems by allowing droplets in this state to be mobile.
To do so, Wong’s team etched micrometer scale pillars into a silicon surface, and then crafted nanoscale textures onto the pillars. Next, they infused the nanotextures with a layer of lubricant that completely coated the nanostructures, resulting in greatly reduced pinning of the droplets and improving the lubricant retention versus the microstructured surface alone.
These same techniques can be used on materials other than silicone including plastics, metals, ceramics, and glass, the authors noted. They believe that their work, which was supported by the National Science Foundation and the Office of Naval Research, could encourage others to search for a new, unified model that can explain wetting phenomena on rough surfaces.
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Feature image: Xianming Dai, Chujun Zeng and Tak-Sing Wong/Penn State
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