R&D INSIGHT
Bouncing droplets powered by evaporation
some of their liquid, according to scientists from ETH Zurich in Switzerland. The bounces become successively higher until the droplets freeze. In the new research, mechanical
W
engineer Dimos Poulikakos and colleagues 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
ater droplets will bounce spontaneously
on a
specially designed surface by simply evaporating
because water droplets will sit on top of the micropillars. This reduces the contact area between the droplet and the surface, making the droplet less likely to stick. At
ambient atmospheric normally pressure, reduces the this
small contact area is sufficient to keep droplets tethered to the surface. Lowering air pressure
water
repellence of a superhydrophobic surface, because with less air underneath it, a drop will sink deeper into the textured surface. However, when Poulikakos and colleagues looked closely at
droplets in one per cent atmospheric pressure, they found them bouncing up and down on the micropillars with ever-increasing height. The unexpected bouncing is related to an
the behaviour of water
additional effect of low pressure – it reduces the humidity of the air, allowing the droplet to evaporate faster. Vapour evaporating from the bottom of the droplet cannot easily dissipate between the micropillars, generating pressure under the droplet sends it flying upwards. The pressure is then released and the droplet falls back down to the surface. The droplet then bounces upwards with an extra kick generated by its vapour pressure. This effect is repeated again and again, with the evaporation of the droplet
feeding increasing amounts
of energy into its motion, causing it to bounce higher each time.
These hands were made for punching
A spider from the same family as the new discovery
US scientists have conducted a bizzare experiment involving the arms of corpses, fishing line and guitar-tuner knobs that has lent support to the theory that human hands are built for punching. Arms from human corpses were placed in a pendulum-like apparatus that allowed them to swing punches at a padded force- detecting surface. To make the dead fists clench, lengths of fishing line were attached to tendons controlling movements of the wrist, thumb and fingers, and guitar knobs were used to tighten or slacken the lines to switch between a “slapping” open hand, a hand formed into a fist with the thumb extended, or a “buttressed” fist with the thumb locked around the index and middle fingers. After hundreds of punches and slaps the scientists concluded that human fists function as efficient clubs. “Our results suggest that humans can safely strike with 55 per cent more force with a fully buttressed fist than with an unbuttressed fist, and with two-fold more force with a buttressed fist than with an open hand slap,” says Professor David Carrier, of the University of Utah. It is widely believed that our shorter palms and longer, stronger and more flexible thumbs compared to other apes helped our ancestors make and use tools, but researchers now think that it may also be down to our ancestors habit of settling differences over mates and resources with bare-knuckle boxing. The arms used were obtained from the university’s body donor programme and a private supply company.
www.projectsmagazine.eu.com
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