Charges make micro whirlpools

One of the classic challenges of making practical labs-on-a-chip is finding ways to easily mix liquids. In the tiny amounts used by biochips, liquids lose the assist that turbulence gives to mixing, making something with the viscosity of water act more like honey. Biochip microchannels can be narrower than a micron, which is five times smaller than a red blood cell.

Researchers from the Hong Kong University of Science and Technology have devised a way to define tiny patterns that carry positive, negative or neutral charge on the surface of a microchannel. Surfaces naturally accumulate electric charges, and oppositely-charged ions in liquids are drawn to the liquid-surface interface. Applying oppositely-charge electrodes to either end of the channel causes these ions to flow, and friction moves the rest of the liquid with them.

The patterned charges make it possible to create different flow patterns inside a microchannel, including very small vortices, or whirlpool-like flows capable of mixing tiny amounts of liquid quickly and within a relatively small space.

The technique could improve microchannel mixing, and can also be used to make other types of flow patterns, according to the researchers.

The mixing scheme should be ready for practical applications within two years, according to the researchers. The work appeared in the November 17, 2003 issue of the Journal of Micromechanics and Microengineering.