Liquid drops defying gravity

Researchers at the University of Bristol, UK, have shown that droplets of liquid can travel uphill when placed on a vertically vibrating inclined plate. 'In fact, if the plate vibrates at the right rate, the droplets will always travel counter-intuitively up the incline.' This very interesting discovery will not change the world, but it may lead to new methods to manipulate microscopic amounts of fluids.

Researchers at the University of Bristol, UK, have shown that droplets of liquid can travel uphill when placed on a vertically vibrating inclined plate. 'In fact, if the plate vibrates at the right rate, the droplets will always travel counter-intuitively up the incline.' This very interesting discovery will not change the world, but it may lead to new methods to manipulate microscopic amounts of fluids.

Liquid drops defying gravity

You can see above different views of droplets of a glycerol-water mixture seen from above. Here are shown sliding (1), static (2), and climbing drops (3-5). As the speed of the drop increases from (3) to (5), the trailing end transitions to a corner (4), and then to pearling. (Credit: University of Bristol)

This research project has been done at the Department of Mathematics at the University of Bristol by Philippe Brunet, Jens Eggers and Robert Deegan.

I'm sure you all want to know how such a thing is possible. Here is the reason. "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 would have the droplet slide down as the plate moved upward. Similarly, the droplet would climb up the incline as the plate drops, resisting the rapid downward acceleration. 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."

This research work has been accepted for publication by Physical Review Letters, one of the journals of the American Physical Society. Here is a pre-publication of this paper, "Vibration-induced climbing of drops" (PDF format, 13 pages, 762 KB), from which the above illustration has been extracted.

Here is the abstract. "We report an experimental study of liquid drops moving against gravity, when placed on a vertically vibrating inclined plate, which is partially wet by the drop. Frequency of vibrations ranges from 30 to 200 Hz, and above a threshold in vibration acceleration, drops experience an upward motion. We attribute this surprising motion to the deformations of the drop, as a consequence of an up/down symmetry-breaking induced by the presence of the substrate. We relate the direction of motion to contact angle measurements. This phenomenon can be used to move a drop along an arbitrary path in a plane, without special surface treatments or localized forcing."

And here are some of the conclusions of the researchers. "Our results suggest a device in which droplets can be moved arbitrarily and in parallel by independently varying the phase and amplitude of the vertical and horizontal vibrations for each axis. Recent studies have demonstrated spontaneous drop motion due to gravity fields, wettability gradients, an interplay between thermal effects and ratcheting, asymmetric vibrations, and chemisorption. By contrast, our transport mechanism would work for uniform substrates, zero mean forcing, and in the absence of external imposed gradients."

Sources: American Physical Society, via ScienceDaily, September 26, 2007; and various websites

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