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Slick Advance in Engineered Surfaces

Thursday, September 3, 2015

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Penn State researchers say they have engineered a nano-textured, highly slippery surface capable of outperforming natural coatings that repel water and dirt.

“This represents a fundamentally new concept in engineered surfaces,” explained Tak-Sing Wong, assistant professor of mechanical engineering and a faculty member in the Penn State Materials Research Institute, in a university announcement released Monday (Aug. 31).

water droplets
Xianming Dai, Chujun Zeng and Tak-Sing Wong / Penn State

This schematic shows a new engineered surface that can repel liquids in any state of wetness.

Lotus leaves and other natural surfaces have long served as a model for liquid-repelling surfaces, but tiny water droplets and vapors still stick to them, the team reported.

The new surface, however, is capable of repelling liquids regardless of the state of wetness, according to the scientists.

Potential Impact

While the team used silicon surface as a basis for the study, the same design could be applied to other materials such as metals, glass, ceramics and plastics.

The advance may also help create more efficient water harvesting in arid regions; improve condensation heat transfer for power-plant heat exchangers; and even prevent icing and frosting on aircraft wings, according to Penn State.

The team recently published its research in the online edition of ACS Nano.

Challenging Feat

The team explains that liquid droplets on rough surfaces come in one of two states: “Cassie,” a state in which the liquid partially floats on a layer of air or gas; or “Wenzel,” in which the droplets are in full contact with the surface, trapping or pinning them.

Lotus leaf
©iStock.com / Noppharat05081977

Lotus leaves and other natural surfaces have long served as a model for liquid-repelling surfaces, but tiny water droplets and vapors still stick to them, the team notes.

Cassie and Wenzel are named after physicists who first described them. While the Wenzel equation was first published in 1936 in a highly cited paper, it has been challenging to verify the equation experimentally, the team notes.

“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,” Wong said.

“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.”

A Slippery Wenzel

In order to make Wenzel state droplets mobile, the researchers said 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.

rough surface
Xianming Dai, Chujun Zeng and Tak-Sing Wong / Penn State

In conventional superhydrophobic rough surfaces, tiny liquid droplets in the Wenzel state will remain pinned to the surface textures. In contrast, the new slippery rough surface enables high mobility for Wenzel droplets.

The nanostructures also greatly enhanced lubricant retention compared to the microstructured surface alone, the team reported.

The researchers believe this work will open the search for a new, unified model of wetting physics that explains wetting phenomena on rough surfaces.

The team has filed a U.S. provisional patent for the discovery.

A National Science Foundation CAREER Award and a Graduate Research Fellowship, and the Office of Naval Research (MURI award) supported the research.

   

Tagged categories: Biomimicry; Coating chemistry; Coatings Technology; Research and development; Self-cleaning coatings

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