Most people experience wrinkling of their fingers and toes when they spend time in water. It was often assume to be caused by water absorption or some other passive effect, but it doesn't happen in people with certain kinds of nerve damage and this points to it being an active process, a reaction to water exposure. The wrinkling it turns out is a useful adaptation as it improves grip when clambering over or holding on to wet rocks, for instance. The ability to control wrinkling of artificial materials could have a range of applications such as anti-fouling surfaces for the hulls of boats, improved underway gripping in robotic settings, controlling wettability of devices and equipment. At a subtler level control of reflectivity and albedo of surfaces for grating applications might be possible, adaptable wearable technology and perhaps even refreshable Braille readers.
Now, an international team has demonstrated electrical control of a polymer coating applied over an electrode array that allows them to induce reversible wrinkling of the coating on the microscopic scale. [Lin, I.T., et al. Mater. Today (2020): DOI: 10.1016/j.mattod.2020.09.028]
The researchers from the University of Cambridge and Queen Mary University of London, UK, and Stanford University, California, USA, explain that reproducible demonstrations of voltage-controlled actuation of thin films has been reported previously. However, to obtain deformation with regular geometric patterning of a film or surface requires more sophisticated control. They hoped to find a way to program a surface to produce very specific changes in its anisotropy at the microscopic level. They have now found a way to print an appropriate array of electrodes that allows such control. The control requires no mechanical bending or stretching components functioning wholly through an electrical response in the polymer. Deformation occurs on a timescale of less than 1/3 of a second in the team's proof of principle.
The demonstration system utilizes a polydimethylsiloxane (PDMS) elastomer layer and a layer of spin-coated Sylgard over a top electrode gold nano-layer with the electrode pattern printed by aerosol jet. The team created an interlocked double-comb electrode pattern and used this to apply the system as a mirror-diffusion-diffraction grating that would thus be a multifunctional device. The patterning of the polymer is determined entirely by the activation of the underlying electrodes addressed individually and the applied voltage and produce complex responses in the device.