14 DAY TRIAL // We know that raindrops falling on our wind screens don't slide down too quickly, while the larger ones roll down the window getting mixed with the smaller ones. This is due to the fact that the surface tension holds the small drops onto the screen, until their increasing size makes the gravitational force bigger than the surface tension.
But what if droplets of water travel uphill, instead of sliding down? This is not an idea taken from a science fiction movie; it's what a team of mathematicians at the University of Bristol has achieved, when they applied the right vigorous up and down shake to the surface. The researchers revealed that the droplets can defy gravity and move up hill, even on a slope as steep as 85 degrees, if the surface vibrates up and down strong enough. "Moving small droplets - such as thousands of spots of DNA arranged on a solid surface (a DNA microarray) - is very difficult as their small size causes them to stick to the surface. So improving our understanding of what causes droplets to move on surfaces will help with this and similar problems," said Dr Philippe Brunet, in the Department of Mathematics.
"As the shaking plate rises the drop is compressed, while it bulges upward as the plate falls. If the shaking is vigorous enough to overcome the surface tension experienced as the drop is compressed, the drop will tend to lean forward, producing a net force which drives the drop uphill." said Professor Jens Eggers, also from the University's Maths Department.
But as the droplet must face a high power, alternately pushing and pulling can expose it to the danger of breaking apart. That's why the droplet cannot overcome a specific size and the liquid has to be a little thicker than pure water.
Tests on droplets of pure water showed it would break apart before the powers are sufficiently strong to push them up, while in the case of the too thick liquids, the movement is too slow.
The new method for displacing droplets employing vibrations could have various applications, such as improving the small-scale manipulation of fluids.