Super-slippery surfaces

22 September 2015

We spoke with Dr Tak Sing Wong in 2011 about his work with super-slippery materials. We recently caught up with Wong (2004 Croucher Fellowship), to hear about the latest developments in the super-slippery world.

Wong is assistant professor of Mechanical Engineering at the Department of Mechanical and Nuclear Engineering at The Pennsylvania State University. Wong’s research combines biologically inspired concepts and innovative material design to develop super-slippery surfaces that can be applied in a broad range of industrial, medical, and environmental applications.

Super-slippery surfaces are exactly what they sound like- a slippery surface material, which allows water, oil and even more viscous substances, such as honey, to slide off without leaving any residue. Imagine being able to get the last drop of ketchup out of the bottle with ease.

Inspiration is often taken from nature in designing super-slippery surfaces. In fact, a lot of progress has been made in developing synthetic liquid-repellent surfaces based on the design principle of lotus leaves. At microscopic scale, the surface of these leaves is not smooth, but consists of a bumpy layer of epidermis cell, which traps a thin layer of air and allows water droplets to roll off easily. A similar principle explains why a puck can glide smoothly on an air hockey table.

Taking inspiration from lotus leaves, engineers have developed a micro-structure surface that traps a thin layer of air, and can repel water droplets. However, using trapped air as a liquid-repellent has a few fundamental limitations: first, trapped air is not an effective cushion against organic liquid, and cannot operate under high pressure conditions that are common in biomedical, environment, and energy applications. In addition, the synthetic micro-texture designed to trap air is prone to irreversible defects- from mechanical damage to fabrication imperfections. These factors have restrict the use of lotus-leaf-inspired surfaces in industrial applications.

Instead of making evolutionary advances based on the lotus leaf design, Wong has turned to nature again to look for new inspiration. He turned his focus to the insect-trapping strategy of the Nepenthes pitcher plant. Instead of using micro-structure to trap air, like the lotus leaves, the pitcher plant used a nano/micro-textured surface to lock in place a thin layer of lubricating liquid, which repels the insect’s oily feet. Using this design principle, Wong developed a pitcher plant inspired surface called “slippery liquid-infused porous surfaces”, while he worked as a post-doctoral researcher at Harvard University. Since liquid is inherently smooth, self-healing, incompressible, and has the capability to repel various immiscible simple and complex liquids, the slippery liquid-infused porous surfaces has demonstrated exceptional liquid-repellency and pressure stability that the lotus-leaf-inspired designs lacked. Furthermore, although in nature, the pitcher plant uses rainwater as a lubricant, in the laboratory, the choice of lubricant can be much more flexible- from water, to oil, etc.- depending on the liquid which the surface is designed to repel.

Soon after its development, engineers saw tremendous potential in slippery liquid-infused porous surfaces. For example, engineers noticed from early on that this kind of surfaces has exceptional anti-biofouling properties, meaning that bacteria cannot stick to, and grow on such surfaces. This makes the surface an attractive potential material for biomedical devices, such as catheters and medical implants. Another interesting application for slippery liquid-infused porous surfaces, is to develop anti-ice and anti-frost coating on aluminum, one of the most widely used lightweight structural materials.

Recently, Wong and collaborators at Harvard University successfully developed materials with anti-ice performance, which are at least an order of magnitude better than other state-of-the-art materials, which has opened up the possibility of using such coating in a broad range of applications, including refrigeration, aviation, roof, or wind turbines.

But innovations from Wong’s laboratory do not just stop at slippery liquid-infused porous surfaces. Recently, researchers in his lab developed a biologically inspired surface inspired by two plant species. The new development, called slippery rough surface, “represents a fundamentally new concept in engineered surfaces… our surfaces combine the unique surface architectures of lotus leaves and pitcher plants, such that these surfaces possess high surface area, and slippery interface, to enhance droplet collection and mobility”, says Wong. This cross-species bioinspired material may have potential applications ranging from condensation heat transfer for heat exchangers in power plants, to water harvesting for drinking water and irrigation for agriculture in arid regions. Ultimately, the broader goal of Wong’s research is to develop technological innovations that can improve the quality of human life.

Wong’s research has received numerous recognitions, including the under 35s Top 35 innovators by MIT Technology Review by MIT Technology Review, the United States National Science Foundation CAREER award, DARPA Young Faculty Award, and the Top 20 Outstanding Alumni in the Department of Mechanical and Automation Engineering at The Chinese University of Hong Kong.

After receiving his undergraduate degree in Automation and Computer-Aided Engineering at The Chinese University of Hong Kong in 2003, Professor Wong went on to the University of California in Los Angeles and completed a Ph.D. in Mechanical Engineering in 2009. Upon completion, he conducted postdoctoral research at the Wyss Institute for Biologically Inspired Engineering at Harvard University from 2010 to 2012. He was a recipient of the Croucher Foundation Fellowship from 2010 to 2012.

To view Dr Tak Wong’s personal Croucher profile, please click here.

To access the Wong Laboratory for Nature Inspired Engineering, click here.