Brussels, Jan 2006
Let a micro droplet dance, and the electric charge on the droplet can be regulated very accurately. During this 'spontaneous oscillation', the droplet touches an electrode periodically. The rhythm of the dance determines the amount of charge on the droplet. This is what scientists prof. Frieder Mugele and Jean-Christophe Baret of the University of Twente (Physics of Complex Fluids group, IMPACT/MESA+) found out. Being able to control the electric charge on a droplet, they foresee applications in inkjet printing, for example, and the fabrication of medical sprays. Another possibility is mixing tiny amounts of fluid. The research has been published in Physical Review Letters.
Mugele and Baret found the dancing microdroplets in a process called electrowetting. From fluid that would normally spread out over a surface, droplets can be 'pulled' using an electric field. Electrowetting is a good way to make tiny droplets, for transporting and manipulating very small amounts of fluid. During this process, the droplet can start to dance. The upper side of the droplet alternately touches an electrode and breaks this contact again. Just before the contact is broken, the droplet forms an extremely thin 'neck'. This is the electrical connection towards the droplet: it determines to what extent the droplet will be charged. The droplet only starts dancing again when no charge is left behind while breaking the neck.
Using these results, it is possible to control the charge on a droplet accurately, for inkjet printing, making medical sprays or for ionizing electrosprays. Electrowetting is used to 'drive' arrays of microdroplets in microfluidics. It is also used in making displays and 'electronic paper'.
The research is done by the Physics of Complex Fluids group - http://pcf.tnw.utwente.nl - of prof. Frieder Mugele. Jean-Christophe Baret, who finished his PhD-work within this group in cooperation with Philips Research, is currently working at CNRS in Strasbourg.