A group of physicists in France has achieved something that others have been unable to do for over 50 years: use electric fields to separate out two mixed liquids. Ludwik Leibler and colleagues at the Ecole Supérieure de Physique et Chimie Industrielles (ESPCI) in Paris did so by applying a non-uniform field across a mixture of paraffin and silicone oil. The method could prove especially useful for separating out liquids on the microscale (Y Tsori et al. 2004 Nature 430 544).
Separating out two liquids from a mixture — such as oil and water or a polymer and its solvent — is a process often employed by industry. It is usually achieved by lowering the temperature of a mixture to its so-called phase transition temperature, at which point the thermal vibrations that naturally sustain the mixing process are minimized. However, there are many situations where it would be useful to use other methods of separation — when it is necessary to keep the temperature constant, for example.
Leibler and colleagues, on the other hand, have shown that non-uniform fields couple up to 50 times more strongly than uniform fields because the coupling is direct. To demonstrate this they placed their mixture in a cell made of two glass slides, the lower slide containing an array of 25 nanometre-thick indium-tin-oxide electrodes. The electrodes were arranged so that they were unevenly separated from each other, which meant they produced a highly non-uniform field.
Using a potential difference of 100 volts, they observed that the silicone oil moved towards the electrodes, while the paraffin remained further away. Moreover, the liquid reverted to its mixed state once the electric field was switched off. Leibler predicts that the effect could also be induced by electromagnetic radiation, such as laser beams, thus opening up the possibility of applications in electro-optics.
