Oil unmixed. When enough voltage is applied, silicone oil separates from a mixture and hovers along a pair of electrodes in a bead a few micrometers wide.

Parting the Oils

An electric field can cause two thoroughly mixed liquids to separate, researchers have shown. The surprisingly simple effect might someday direct the flow of fluid through tiny capillaries in high-tech "mircofluidic" devices or control video displays.

Oil and water may never mix, but many combinations of two liquids aren't so strict. Above a critical temperature, they'll mix. Below that temperature, they'll only mix in lopsided proportions. Try to stir up a more balanced concoction, and the combination spontaneously separates into two "phases"--one consisting primarily of the first liquid, the other consisting primarily of the second. Such phase separation explains why anise-flavored liqueurs such as pastis and ouzo go cloudy when diluted with water; the cloudiness actually consists of tiny droplets of one phase suspended in the other.

For decades researchers have tried to control such mixing and separation by, among other things, applying electric fields. If the field is spatially uniform, it must be extremely strong--stronger than the fields that drive lightening through air--and the mixture must be within a few hundredths of a degree of the critical temperature. However, an uneven electric field neatly pulls the liquids apart at lower field strengths and a wider range of temperatures, physical chemist Ludwik Leibler and colleagues at the City of Paris Industrial Physics and Chemistry Higher Educational Institution (ESPCI) report in the 29 July issue of Nature.Using a pair of indium tin oxide electrodes painted onto a glass slide, the researchers separated a mixture of silicone oil and paraffin derived from shark liver oil. The technique works because the two liquids react differently to an electric field. The silicone oil is more easily "polarized" and migrates toward the places where the field is strongest, displacing the paraffin. When the field reached a certain strength, the mixture suddenly separated into two phases, with the silicone-rich phase lining the edges of the tiny electrodes. The same approach should work for a variety of liquids."It's one of those things that once you see it, it seems obvious," says David Grier, a physicist at New York University. "But until you see it, obviously it's not." Pierre Wiltzius, a physicist at the University of Illinois, Urbana-Champaign, says that if researchers can further reduce the voltages and increase the temperature range, the effect might be useful in a wide variety of micrometer-scale technologies. "This is not pie in the sky," Wiltzius says, "this has real potential for applications."

Related sites
ESPCI's Soft Matter and Chemistry Lab
The Wiltzius group home page
The Grier group home page