Jun 17 2019
Self-cleaning surfaces and laboratories on a chip can become a lot more efficient if individual droplets can be controlled. University of Groningen professor Patrick Onck, along with colleagues from Eindhoven University of Technology, has demonstrated that this is possible by using a method known as mechanowetting.
“We have come up with a way of transporting droplets by using transverse surface waves. This even works on inclined or vertical surfaces.” The study was published in Science Advances on June 14th.
A glycerol droplet travels along with the wave. Small particles in the droplet visualize the internal fluid flow. (Illustration De Jong et al., Sci. Adv. 2019;5: eaaw0914)
The concept of mechanowetting is essentially very simple: place a droplet on a transverse surface wave and the droplet will travel with the wave. “One of the properties of water droplets is that they always try to stay on top of a wave. If that top runs ahead, the droplet will run with it," Onck explains. It is possible to transfer the droplets by using mechanical deformation to form surface waves. “The remarkable thing about this is that it also works on inclined or vertical surfaces: drops can even move upwards against gravity.”
Theory
Edwin de Jong, PhD candidate in Onck’s group and the paper’s first author, tested the idea of mechanowetting using a computer model. “When it seemed to work in theory, our colleagues from Eindhoven University of Technology devised an experiment to test it. Our model turned out to be right: in practice, the drops moved exactly as we had imagined.”
One of the applications of mechanowetting is in lab-on-a-chip systems, whole laboratories the size of a credit card, which are used to examine biological fluids such as saliva or blood.
Samples can now be tested outside the lab, for example, directly at the bedside, with a lot faster response rate. “If we are able to direct each drop separately, it is possible to perform a lot of different tests at high speed with a very small volume of fluid," says Onck. Moving droplets individually was already possible by means of electrowetting.
Electrowetting is able to transport droplets by applying electric fields. However, these fields can change the biochemical properties of the sample, and that is something you don't want when doing blood tests.
Patrick Onck, Professor, University of Groningen
Light
In the interim, Onck’s team is investigating new prospects. “We have performed computer simulations that show that mechanowetting also works by using light-responsive materials to create waves. Light is especially interesting because of its precision and its ability to control the movement of drops remotely.”
Besides lab-on-a-chip systems, mechanowetting has a number of other fascinating applications such as self-cleaning surfaces where water droplets actively absorb and eliminate the dirt. It also provides opportunities for harvesting moisture from the air, by gathering dew drops for use as drinking water.