Monday, 5 September 2011

A motor made out of thin films

Adding wires to liquid films

Last year, a group of physicists from Tehran made the discovery that motors can be made of nothing more than a thin film of water sitting in a cell bathed in two perpendicular electric fields. The unexpected result of this set up is that the water begins to rotate. If one divide the water into smaller cells and each rotates too.
Check out the videos from the team at the Sharif University of Technology in Tehran  a number of  fascinating videos of it in action

What is making the thin water move? 

Vlad Vladimirov at York University in the UK asked the same question and they delved into the hydrodynamics to work out why this water motor is working. The key turns out to be the scale on which the effect takes place.  Since it is only a thin film of water as you can see from the videos the electrical current and the surface tension allow it to rotate.  (Not sure if it is just the top film that is rotating or the whole body of water.  Also it probably depends on what kind of film they added.).  They say the flow is generated at the edge of the cell where the electric field crosses the  (dielectric) boundary between the water and the cell container. The change in field sets the water flowing along the boundary.  Crucially, this flow is opposite on the other side of the cell and this is what sets up the circular flow.

Vladimirov points out that this effect can only happen in a thin film of the dimensions that they used in Tehran where effects such as viscosity and friction play a large role in the dynamics. In larger bodies of water, these effects become insignificant and the rotation stops. So scale depends on this kind of motor so no seeing this in the ocean.  However, this might be very interesting for its use in microfluidic devices and for parties. 
Ref: Rotating Electrohydrodynamic Flow in a Suspended Liquid Film