Physicists develop new methods to manipulate miniscule droplets

The researchers from the Physics of Complex Fluids group of the University of Twente are world leaders in the area of 'electrowetting'. With this technique you can deform small droplets and set these in motion with the help of an external electrical field. This technology is used in many areas, for example in lab-on-a-chip technology, optofluidics and display technology.

Marble run
In the research published on 11 April, the researchers reversed the method. They demonstrated that you can also use electric fields to slow down droplets or completely stop these.

In their experiments the researchers allowed droplets to glide downwards over an inclined surface that was strongly hydrophobic. When they applied a local electric field to the surface they managed to slow down or even completely stop the droplets (see film). Professor Frieder Mugele, who was involved in the research, compares it to a marble run: "In a marble run, the marbles initially accelerate. If you place a cavity in the track that is deep enough and there is sufficient friction then all of the marbles will eventually be stopped. In our research we did exactly the same. However in this case a cavity is not a physical cavity but a 'potential well' generated by a variable voltage difference." The researchers produced a scientific model that can accurately describe the observed behaviour of the droplets .

Targeted manipulation
With this new method the scientists can very specifically manipulate small droplets. This is ideal for microfluidic systems, such as labs-on-a-chip (small laboratories the size of a chip) that are used, for example, to perform blood tests. The method can also be used to separate droplets that contain a cancer cell from droplets that contain another cell. A big advantage of the method is that a high throughput of droplets can be achieved, while still allowing the individual droplets to be accurately manipulated (see film). The scientists demonstrated that earlier this year in a publication in the journal Lab-on-a-Chip. The research could also be used, for example, to clean chips in the semiconductor industry and for research into methods to extract more oil from existing oilfields.

Research
The research was carried out within the FOM Industrial Partnership Programme (IPP) 'Contact Line Control During Wetting and Dewetting' by researchers from the Physics of Complex Fluids group of the MESA+ Institute of the University of Twente. For this research the group worked closely with researchers from the Mesoscopic Transport Properties group of Eindhoven University of Technology. The research was partly funded by the FOM Foundation, ASML and Océ.

Contact
Joost Bruysters, science information officer University of Twente, +31 (0)53 489 2773

Reference
Trapping of drops by wetting defects , Nature Communications, 11 April 2014 (open access)