Researchers at Aalto University in Finland have discovered a novel way to write and present information using only water and air. They used the water-repelling properties of the lotus leaf as inspiration for an experiment with a superhydrophobic (“water-repelling”), dual-scale surface that allows the writing, erasing, rewriting and storing of optically displayed information in plastrons related to different length scales. The research was carried out in partnership with the Nokia Research Center and University of Cambridge and was led by Dr. Robin Ras at Aalto University.
The surface was placed inside a container filled with water and featured microposts of ten micrometers (approx. 0.0004 inch) in size and tiny nanofilaments grown on the posts. This type of two-level surface allows the air layer to exist in two different shapes that correspond to the two size scales. Using a nozzle, the scientists succeeded in switching between dry and wet states by creating excessive or insufficient pressure in the water in order to change the air layer to either state. The switching only involves a change in the shape of the air layer while nothing actually happens to the solid surface itself, allowing them to write shapes on the surface underwater by making use of the contrast between the states. It can be done with precision, pixel-by-pixel. The whole “screen” then can be deleted by removing it from water. The surface comes out dry, with no sign of writing on its surface.
The lotus leaf has been a source of inspiration for several research projects, thanks in part to its structure which grants it its remarkable water-repellence properties. What this new research reveals is that there is a strong optical contrast between dry and wet states thanks to a difference between the microscopic structures and the nanoscopic structures on the surface of the former, which enables a clear display of information.
“Because each wetting state remains stable (no energy is needed to maintain the wetting states, only to switch between them), such surface can also act as a bistable memory that can be easily readable, even by eye,” Dr. Rai told Gizmag.
An in-depth article entitled Reversible switching between superhydrophobic states on a hierarchically structured surface appeared in PNAS last week.
The video below illustrates the experiment.
Source: Aalto University.