Fluid frenzy at MIT

In separate studies, two groups of researchers from Massachusetts Institute of Technology have been probing the properties of fluid droplets.

Firstly, researchers unexpectedly discovered water droplets that form on a superhydrophobic surface before jumping away carry an electric charge. Under certain conditions, rather than simply sliding down and separating from a surface due to gravity, droplets actually leap away from it.

This occurs when water droplets condense onto a metal surface with a specific kind of superhydrophobic coating and at least two of the droplets coalesce. They spontaneously jump from the surface, resulting in the release of excess energy.

“We found that when these droplets jump, through analysis of high speed video, they repel one another mid-flight,” said postdoc Nenad Milijkovic. “Previous studies have shown no such effect. When we first saw that, we were intrigued.”

Using a charged electrode, researchers performed a series of experiments to better understand why this happened. When positively charged, droplets were repelled by the electrode and each other, when negatively charged they were drawn to it – researchers concluded a net positive charge forming on the drops as they jumped away from the surface.

The charge is the result of an electric double layer on the surface of the droplets when they formed. When neighbouring drops meet and jump from the surface, the process happens so fast that the charge separates, leaving some on the droplet and the rest on the surface.

The findings – published in Nature Communications – could provide a way of enhancing the efficiency of heat transfer on condensers in electricity-generating power plants, or a new way of drawing power from the atmosphere.

In a second study, researchers from the Department of Mathematics examined the dynamics of fluid droplets walking on a vibrating fluid bath. These droplets exhibit certain features thought to be exclusive to the microscopic quantum realm. Using a theoretical model they described the dynamics of bouncing and walking droplets, and developed an equation which is still under exploration.

“Of course, if we ever hope to establish a link with quantum dynamics, it’s important to first understand the subtleties of this fluid system,” said John Bush, professor of applied mathematics. “Our next step is to use this equation to better understand the emergence of quantisation and wave-like statistics, both hallmarks of quantum mechanics, in this hydrodynamic pilot-wave system.”

Electrostatic charging of jumping droplets http://www.nature.com/ncomms/2013/130927/ncomms3517/full/ncomms3517.html

Exotic states of bouncing and walking droplets http://pof.aip.org/resource/1/phfle6/v25/i8/p082002_s1?bypassSSO=1

http://www.youtube.com/watch?v=0T66p8ocA7w

http://www.youtube.com/watch?v=9iRiktsE5w0