Scientists in Finland and the UK have created a surface with tunable superhydrophobicity and used it to create a display that can be written on with water. Structural hydrophobicity, as seen with lotus leaves, uses microstructures to prevent wetting and instead keeps a layer of air between the surface and the water, known as a Cassie state. For example, many underwater insects use hairs to repel the water and keep a layer of air trapped between them and the bulk water.
In Proceedings of the National Academies of Science the team, led by Robin Ras at Aalto University in Helsinki, made a two level topography, with both a micro and nano structure, that allows two different Cassie states to exist. When the surface is submerged, the normal, micro-Cassie state exists but if the water pressure is increased, for example by a small jet of water directed onto the surface, the air is forced down to the next tier of structure, creating what the researchers call a nano-Cassie state. Reducing the pressure with suction reverses the transition.
These two stable wetting states allow binary information storage, and because of the different reflectances of the states, give a visual readout.
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