'Jumping' water droplets could lead to better solar panels

Researchers from Duke University in North Carolina believe they have found a way to make certain types of thermal diode more practical for applications such as energy-efficient solar panels and buildings that adapt to outside temperatures.

Thermal diodes are devices that control the flow of heat by effectively creating a thermal conductor in one direction and an insulator in the reverse direction.

‘Phase-change’ thermal diodes use vaporisation and condensation to transport heat and reportedly transfer more than 100 times more heat in the forward direction than the reverse, but they are dependent on gravity and restricted by their tubular configuration.

The Duke University engineers believe they have figured out a way to overcome these limitations using self-propelled water droplets, which can jump from a superhydrophobic (highly water-repellent) surface to a superhydrophilic (highly absorbent) surface, but not the other way around.

Chuan-Hua Chen, assistant professor of mechanical engineering and materials science at the university’s Pratt School of Engineering, and his research group were the first to actually videotape the self-propelled jumping motion of water droplets on a superhydrophobic surface.

They found that the droplets literally jumped straight up and off the surface — overcoming the gravity limitations of typical phase-change diodes.

‘When the superhydrophobic surface is colder than the superhydrophilic surface, the heat transport is very effective with phase-change processes, much like sweat taking away body heat,’ said Chen.

‘When the superhydrophobic surface is hotter, the heat flow is blocked and the diode behaves like a double-paned window.

‘Because the jumping droplets in our system are very small, gravity has a negligible effect on them. Therefore, devices based on this approach can be oriented in any direction without the need to worry about gravity.’

The results of the Duke University experiments were published online in the journal Applied Physical Letters.