Moving water is fairly straightforward on the human scale: a pump or a bucket will usually do the trick. But in the last couple of decades, various teams have begun to study ways of moving liquids around on the much smaller scale of micrometres.
Their goal is to create devices, such as a lab-on-a-chip, that can carry out self-contained chemical and biological tests on tiny samples. To that end, researchers have developed various new ways to move liquid around using exotic pumps relying on things like electric fields. So-called microfluidic devices are having a big impact in areas from pathogen identification to environmental monitoring
Last year, Steve Arscott at the The University of Lille in France added another tool to this armoury. He showed that light could modify the wetting angle of a conducting droplet sitting on an insulated conductor.
This system is essentially a capacitor: one conductor separated from another by an insulating layer. Physicists have known for some time that changing the voltage in such a capacitor sets up a force that alters the wetting angle of the droplet. This effect, known as electrowetting, is the basis for various kinds of electric pumps.
Photoelectrowetting works in a similar way, says Arscott. With a voltage across the capacitor, the incident light generates charge pairs within the droplet that influence the electric field in the capacitor and this changes the wetting angle.
That was an interesting advance because it raised the prospect of pumping small volumes of water using light and very little power (since there is almost no flow of current).
Today, he and his pal Matthieu Gaude put the photoelectrowetting effect into action. These guys have made a cantilever sitting above an insulated conductor and placed a droplet of water between them so that it fills the gap by capillary action (see above).
Zapping this system with light changes the wetting angle the droplet makes with the cantilever and the electrode below. This makes the droplet thinner, pulling the cantilever down.
The ability to actuate at a distance using light alone could have many applications because it eliminates the need for the complex circuitry and pumps now used to transport droplets. It could also allow optical addressing of autonomous, wireless sensors.
Incidentally. that’s not unlike the light actuation of metamaterials we looked at yesterday. Perhaps there’s a new era of light actuation ahead.