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Surfaces and interfaces

Surfaces and interfaces

Bouncing droplets powered by evaporation

04 Nov 2015
Bouncing along: the time sequence of one droplet

Water droplets will bounce spontaneously on a specially designed surface by simply evaporating some of their liquid, according to scientists in Switzerland. The bounces become successively higher until the droplets freeze. The researchers believe the effect could be put to work on self-cleaning surfaces.

The bouncing is reminiscent of the well-known Leidenfrost effect, whereby a droplet of liquid can levitate slightly above a hot surface such as a frying pan. The high temperature underneath the droplet evaporates some of its liquid to create a vapour pressure that offsets the downward pull of gravity and keeps the droplet afloat. In the Leidenfrost effect, the droplet oscillates briefly with decreasing amplitude before reaching an equilibrium height where the pressure differential is just enough to balance gravity.

In the new research, mechanical engineer Dimos Poulikakos and colleagues at ETH Zurich looked at how water droplets interact with superhydrophobic surfaces at low pressure. The researchers created arrays of tiny silicon micropillars coated with water-repellent fluorosilane. Such a material is expected to be superhydrophobic because water droplets will sit on top of the micropillars. This reduces the contact area between the droplet and the surface, and so makes it more likely that the droplet will not stick.

Pressure drop

At ambient atmospheric pressure, this small contact area is sufficient to keep droplets tethered to the surface. Lowering the air pressure normally reduces the water repellence of a superhydrophobic surface. This is because with less air underneath it, a drop will sink deeper into the textured surface, thereby increasing the contact area. However, when Poulikakos and colleagues looked closely at the behaviour of water droplets in 1% atmospheric pressure, they found them bouncing up and down on the micropillars with ever-increasing height (see image). “I was very surprised, of course,” says Poulikakos.

The researchers have worked out that the bouncing is related to an additional effect of low pressure – it reduces the humidity of the air and thereby allows the droplet to evaporate faster. Liquid evaporating from the top of the droplet dissipates rapidly in the air. Underneath the droplet, however, things are different. Vapour cannot easily dissipate between the micropillars, and this generates pressure under the droplet that increases until it is large enough to detach the droplet and send it flying upwards. Once the droplet leaves the surface, the pressure is released and the droplet falls back down onto the surface. The droplet then bounces upwards with an extra kick generated by its vapour pressure. This effect is repeated again and again, and the evaporation of the droplet feeds increasing amounts of energy into its motion, causing it to bounce higher each time.

Supercool effect

Evaporation also causes the droplet to cool rapidly, putting it into a supercooled-liquid state. When the droplet does eventually freeze, the decrease in enthalpy in passing from liquid to solid causes a sudden increase in its temperature, and a consequent increase in how much vapour it sheds. The sudden increase in vapour then propels the ice crystal away from the surface.

“If the surface is vertical, it will fall off,” says Poulikakos. “If a breeze blows, it will blow it off.” Even if an ice crystal falls back down after it freezes completely, it will be much easier to remove than the crust of ice that would form if water froze on the surface, he explains.

Physical chemist Doris Vollmer of the Max Planck Institute for Polymer Research in Germany is impressed. “It’s beautiful and very amazing work – I would never have guessed this,” she says. She cautions that, at present, this is not a practicable proposal for a self-cleaning surface because it does not work at atmospheric pressure, but she concludes that “the aim of science is fundamental understanding and, from the point of view of fundamental understanding, it’s brilliant”.

The research is published in Nature.

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