Droplets defy gravity to roll uphill

Scientists at the University of Bristol in the UK have shown that liquid drops can defy gravity and roll uphill under certain conditions.

London, Sept 25 : Scientists at the University of Bristol in the UK have shown that liquid drops can defy gravity and roll uphill under certain conditions.

Droplets of liquid on an inclined plate that is shaken up and down can travel uphill rather than sliding down.

In fact, if the plate vibrates at the right rate, the droplets will always travel counter-intuitively up the incline, the scientists say.

Researchers say the reason has to do with pushing and pulling.

As the plate rises, it pushes the droplet upward, and as it falls, it pulls the droplet down.

Inertia forces the droplet slide down as the plate moves upward. Similarly, the droplet climbs up the incline as the plate drops, resisting the rapid downward acceleration.

The scientists, however, say the forces that hold the droplet to the plate get stronger as the plate rises.

During the time that the droplet is moving downhill, it is stuck more firmly to the plate.

Therefore, the droplet gains more ground moving up the incline as the plate falls than it loses as the plate rises. Overall, the droplet travels uphill.

According to a report in the New Scientist magazine, if the vibration doesn't apply enough force to the droplet, it will just sit still on the inclined plate.

"As the force increases, the droplet will begin to slide. Increasing the forces further, the droplet sits still again. Turn up the force on the droplet a little more, and it starts to climb," the scientists said in their report.

The scientists say since the droplet must withstand a fair amount of force, alternately pushing and pulling, the fluid has to be somewhat thick or viscous.

Pure water droplets will break apart before the forces are strong enough to cause them to climb. On the other end, the drops would roll very slowly if the fluid were too thick.

The study appears in the forthcoming issue of the Physical Review Letters

ANI