Writing with water on a superhydrophobic surface immersed under waterAalto University

Applied physicists at Aalto University in Finland have developed optical displays that use only air and water to present information.

The team drew inspiration from the lotus leaf's uncanny ability to repel water. So hydrophobic are the leaves that water droplets simply roll off the surface, leaving them clean from dirt. The leaves' water-repelling ability comes from a thin layer of air trapped by the nanostructure of the surface which the water slides over.

In order to mimic this, a combined team from Aalto University, Cambridge University and the Nokia Research Centre in Cambridge created a silicon surface with similar structures. Two different scales of microstructure were used -- posts of around ten micrometres in height, and tiny nanofilaments that are grown onto the posts.

After that surface has been immersed in water, the air remains. However, it can be disturbed by a jet of water, as seen in the video embedded in this post. By firing the jet at the surface with a nozzle, the air layer gets thinner, changing its reflective properties and allowing the surface to be written on. It can be reversed by sucking water out, reducing the pressure and allowing the air layer to thicken again.

"The minimal energy needed to switch between the states means the system is bistable, which is the essential property of memory devices, for example", researcher Robin Ras explains. However, more interesting is the striking optical contrast between the two states, created due to a change in the roughness of the water-air interface. "Combined with the optical effect, the surface is also a bistable reflective display."

Aalto graduate student Tuukka Verho developed the nozzle used for manipulating the air layer and found that the reversible switching of states could be done with pixel-by-pixel precision. Until now, lotus leaf-inspired surfaces have been developed mostly for self-cleaning, anti-icing or flow drag reduction purposes.

The research has been detailed in a paper published in PNAS.


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