Scientific Method / Science & Exploration

Shaking a fluid uphill

New research demonstrates that one can move a fluid droplet uphill by shaking …

It is easy to get a fluid to move downhill relative to where it begins; getting it up hill can pose more of a challenge. Typically one would use a pump to pressurize the fluid so that it can over come the elevation difference. Before the advent of the various types of mechanical pumps, one could use an Archimedes' screw to move fluid from low lying areas to desired higher elevation destinations. It has also recently been demonstrated that water under a high voltage can defy gravity. Now, a new method gives a way to move fluids uphill without pumps or screws, just shakes. Research from a team of mathematicians from the University of Bristol demonstrates how one can move a droplet of fluid uphill simply by shaking the surface on which the fluid is resting.  

When a droplet sits on an inclined surface, the force of gravity will pull it down. This typical response can be countered by a phenomena known as contact angle hysteresis—when the edge of the droplet on the downhill side contacts the surface in a different manner than the uphill edge. This can result in a capillary force that counteracts the force of gravity and holds the droplet in place. Philippe Brunet has shown that not only can the shape of a droplet hold it in place on an inclined plane but, by deforming the droplet through shaking, it can be made to roll up hill.

In a paper set to be published in an upcoming edition of Physical Review Letters, the researchers show that glycerol-water droplets can actually roll uphill when one vibrates the surface they are on. The researchers have a set of four movies illustrating various aspects of this phenomena available on the author's homepage. They propose that this motion is due to a combination of non-linear effects of friction between the fluid drop and the substrate, and a symmetry breaking during the acceleration cycle present during the shaking. In addition to simply moving uphill, the authors suggest that, by independently controlling the phase an amplitude of horizontal and vertical vibrations, one could force a droplet to move in an arbitrary path along a surface. This aspect the work could lead to improvements in microfluidic devices—in these devices control over where the fluid moves is of the utmost importance.

Physical Review Letters 2007, to be published

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