Using waves to move droplets

June 17, 2019
A glycerol droplet travels along with the wave. Small particles in the droplet visualize the internal fluid flow.

Self-cleaning surfaces and laboratories on a chip become even more efficient if we are able to control individual droplets. Eindhoven University of Technology (TU/e) shows in coordination with the University of Groningen that it’s possible by using a technique named mechanowetting. Together they have devised a way of transporting droplets by using transverse surface waves. This even works on inclined or vertical surfaces. The research was published in Science Advances on 14 June.

The idea of mechanowetting is basically very simple: put a droplet on a transverse surface wave, and the droplet will move 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’, explains Patrick Onck, professor of the University of Groningen. It is possible to move the droplets by using mechanical deformation to create 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 works in practice

Edwin de Jong, PhD candidate in Onck’s group and first author of the paper, tested the concept of ​​mechanowetting by means of 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.’

 

Demonstration of an active, self-cleaning surface. The droplets pick up the dirt particles as they travel along with the surface wave.

Lab-on-a-chip

One of the applications of mechanowetting is in lab-on-a-chip systems, complete laboratories the size of a credit card, which are used to analyze biological fluids such as blood or saliva. This allows the samples to be tested outside the lab, e.g. directly at the bedside, with a much 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. Transporting droplets separately 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.’

 

Barry van der Meer
(Science Information Officer)