However, the forces that hold the droplet to the plate are stronger as the plate rises. During the time that the droplet would be 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 droplets travel uphill.
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.
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 move very slowly if the fluid is too thick. Nevertheless, this method for moving droplets using vibrations may prove useful in the manipulation of microscopic fluids. -KM
Solving a Dragonfly Flight Mystery
Z. Jane Wang and David Russell
Physical Review Letters (forthcoming)
Dragonflies adjust their wing motion while hovering to conserve energy, according to a Cornell University study of the insect's flight mechanics. The revelation contradicts previous speculation that the change in wing motion served to enhance vertical lift.
The Cornell physicists came to their conclusions after analyzing high speed images of dragonflies in action. The insects have two pairs of wings, which sometimes move up and down in harmony. At oth
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American Physical Society