Last year, a group of Iranian physicists made the extraordinary 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. Divide the water into smaller cells and each rotates too.
The team at the Sharif University of Technology in Tehran have a number of fascinating videos of it in action. They raise an interesting question: the electric fields are static, so what’s making the water move?
Now Vlad Vladimirov at York University in the UK and a couple of droogs from Russia have delved into the hydrodynamics to work out what’s putting the oomph in this motor. The key turns out to be the scale on which the effect takes place.
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 and co point out that this effect can only happen in a thin film 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. Which is why these motors have only ever been seen in thin films.
That has important implications becaue it shows the scale dependency of important phenomena. In fact, liquid film motors may turn out to be a game-changers for anybody involved in microfluidics.
Ref: arxiv.org/abs/0902.3733: Rotating Electrohydrodynamic Flow in a Suspended Liquid Film