Here’s an interesting effect discovered by a group of Iranian physicists at Sharif University of Technology in Tehran, Iran (it’s not often we hear from these guys).

They placed a thin film of water in a square cell and applied two perpendicular electric fields. One was an external electric field. For the other, they used two copper electrodes to generate a voltage across the cell like an electrolysing cell (although no chemical reaction took place).

So they had a pair of electric fields at right angles acting on this thin film.

The unexpected result is that the film of water begins to rotate. The team has a number of movies of the effect on its website. They call it a liquid film motor and it’s a quite extraordinary effect. At one point they divide their cell into nine smaller ones and the liquid in each cell rotates in exactly the same way.

The question is: what’s causing the rotation? The team can easily control the direction and speed of rotation by varying the relative angle and direction of the electric fields, which rules out the possibility that convection is causing the rotation (something that is seen when a field is applied to some thin films of liquid crystals). Neither does adding salt to water change the effect, ruling out the possibility that ion movement directs the flow.

The rotation occurs in polar liquids but not in non-polar ones so the intrinsic dipole moment of the molecules seems to be crucial. People have been observing the electrohydrodynamics of various types of thin films for a good few years but nobody has seen anything like this. Just what’s going on remains a mystery.

But the puzzle shouldn’t overshadow what looks like an important discovery that could have widespread industrial application in microfluidic devices for mixing.

Ref: arxiv.org/abs/0805.0490: A Liquid Film Motor