Teflon vs superomniphobic surface. Courtesy UCLA.

Surface tension takes on a whole new significance at the nanoscale. While in bulk, liquids need vessels to contain them, at smaller scales surface tension becomes so substantial that an ant, which can carry 50 times its own mass, can be trapped in a water droplet, unable to break through the surface tension, as illustrated in Kim’s opening slides.

A landmark development in device engineering exploiting surface tension came from HP Labs in the mid-1980s. Kim, a professor at the University of California in Los Angles, US, told Nano Korea 2016 attendees how some inkjet companies were looking for check valve designs to control the release of ink at printing nozzles. Trying to develop microelectromechanical systems to meet these needs proved ultimately impractical, but the surface tension of a bubble could conveniently block, burst and release the ink, providing cleaner, faster inkjet printing.

For many years the contact between a droplet and a surface was primarily understood by considering the chemicals comprising them, so that engineering a surface to be superhydrophobic (repel water) or superoleophobic (repel oil) was a matter of chemistry. However, despite efforts to develop superomniphobic surfaces that would repel everything, there had been no success in developing a surface to repel the liquid with the lowest surface tension of all, the fluorinated solvent FC-172. An alternative approach to pure chemistry provided the solution.

Repellent structures

Improvements to nanofabrication and patterning capabilities have attracted a number of groups to investigate how nanostructures could enhance chemical superhydrophobic properties. However, Kim and his team focused their attention entirely on mechanical effects. To develop a surface that caused droplets of low-surface tension liquids to bead up and roll off, Kim and his team began from the premise of a microscale array of posts.

Liquids where the contact angle – the angle between the tangent to the meniscus and the normal to the surface – is greater than 90° will be naturally repelled by such structures. To repel liquids with lower surface tension, flat ledges were incorporated. These function as a surface at right angles to that of the post, reorienting the contact angle with the net effect of repelling the droplet.

For liquids with a contact angle close to zero, Kim and colleagues added an additional overhang – like a nanoscale serif on a microscale letter T. With these structures the additional re-orientation of the contact angle is sufficient to repel even FC-172. “This is very interesting to me because it is the first mechanical superhydrophobic surface – it has no bearing to chemistry,” Kim told the plenary attendees.

Low drag boats

The nanoserifs on the structures devised by Kim and his UCLA colleagues have an impact on behaviour at the much larger scales of a droplet. The resulting superomniphobic surfaces have applications in self-cleaning and anti-corrosive surfaces, as well as fundamental research.

However, the team have since been looking to apply different kinds of repellant surfaces at a larger scale still. They exploit the lubricity of the air trapped beneath droplets to minimize the drag on boats. “This is nanostructures applied in metre-size applications,” said Kim.

“Drag reduction for large-scale applications such as boats does not need a superomniphobic surface since water is easy to repel,” he tells nanotechweb.org. “Instead, the drag reducing surfaces require additional functions that are different from those of the superomniphobic surface.”

Kim’s talk was presented at Nano Korea 2016, which took place in Seoul July 13–15 2016.