Video: Watch as these droplets zip towards the heated spots on a plate. The secret: the drop isn’t directly on the plate. It’s on a shallow pool of liquid covering the platform.
What you’re seeing is no ordinary plate. It’s supported by titanium micro-heaters and it’s covered with a pool of liquid. The heaters are used to heat up small spots on the plate. The heat radiates through the pool of liquid until it gets to a single droplet of silicon-based liquid — chosen because it won’t mix with the liquid already on the plate.
What happens then depends on the shape of the droplet. Lens-shaped droplets will be boosted away from the hot spot, but more spherical droplets will slowly slide towards the source of heat. The mechanism that drives it is called “thermocapillary migration.” Capillary action — the tendency of liquid to adhere to the walls of certain containers coupled with the tendency of liquid molecules to stick together — is responsible for getting water from the root of a tree to its branches, as well as drawing liquid up the fibres of a paintbrush. Now it’s being used more selectively.
This isn’t the first time that researchers have used thermocapillary migration. Scientists have been able to steer droplets around an exposed plate, but the liquid covering this plate allows them to do it with less of a temperature fluctuation and more speed.
[Source: Droplet actuation on a liquid layer due to thermocapillary motion: Shape effect]