There are a variety of factors that determine the relationship between a water droplet and any given surface. Scientists are however most interested in how surface texture and hydrophobic lubrication can manipulate the overall mobility of water droplets.
Previously, lotus flower leaves and other natural dirt- and water-repelling surfaces have acted as models for engineering synthetic materials like Teflon and other plastic coatings. But recently, researchers at Penn State's Material Research Institute have developed a nano/micro-textured, hyper-slippery surface that works better than its nature-inspired counterparts.
Enhancing the mobility of water droplets (or even vapor) is more important than one might initially think. The science is fairly straightforward, and the applications, various. Increased movability on rough surfaces has the potential to improve condensation heat transfer for power-plant heat exchangers, create a more efficient water harvesting system for especially arid regions, prevent over-icing and frosting on aircraft wings, and protect medical implants from gunk that can build up and ruin the material.
"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," said Tak-Sing Wong, assistant professor of mechanical engineering at Penn State, in a blog post. "We have demonstrated for the first time experimentally that liquid droplets can be highly mobile when in the Wenzel state."
Liquid water droplets interact with any given surface in two states: the Cassie state, where the liquid partially floats on a layer of air or gas, and the Wenzel state, where the droplet is in full contact with the layer of air or gas, trapping it to the surface (and restricting mobility).
The Penn State researchers were able to make a transition from the Wenzel to the Cassie state by altering the surface composition. They etched micrometer scale pillars into a silicon surface using "photolithography and deep reactive-ion etching," after which they produced a microscopic texture on the pillars by wet etching. Then, by infusing a layer of lubricant into the nanotextures, they were able to reduce this trapping of droplets.
The design principle can be extended to various materials as well, beyond simply silicon. These include metals, ceramics, plastics and glass. For the researchers involved, this is a significant first step in a "unified model of wetting physics that explains wetting phenomena on rough surfaces."
