Lasers move droplets

Labs-on-a-chip promise inexpensive and portable biological and chemical analysis. Key to making the tiny labs work is finding ways to move and mix minuscule amounts of substances.

Scientists have devised several methods of manipulating droplets, including moving them with light and electric fields, and using heat to change surface tension. Light, or laser tweezers, methods require droplets to be contained in membranes. Electric field and heat methods require surfaces with patterned electrodes.

Researchers from SRI International have demonstrated a relatively simple method of manipulating droplets that takes advantage of the thermal Marangoni effect: when a droplet is warmer along one side than the other, it shifts toward the cooler side to minimize surface energy. Heat from a laser changes droplet surface tension, which causes droplets to move away from the laser beam.

The method could eventually be used for high throughput biological and chemical screening, and fluidic, chemical and biological experiments that use minuscule amounts of samples and chemicals, according to the researchers.

The researchers demonstrated the method using drops of dyed water on a film of oil on a Petri dish. They were able to move individual droplets ranging from 30 microns to 1,500 microns in diameter as fast as 3 millimeters per second. A micron is one-thousandth of a millimeter. The method can also quickly mix droplets. The researchers mixed a 187-microliter droplet of dye and a 182-microliter droplet of black ink in 33 millionths of a second.

Because the method requires no electrical circuitry, devices can be made cheaply and droplets can move anywhere on the surface, making labs-on-a-chip easy to reconfigure.

The method could be used practically within a couple of years, according to the researchers. The work appeared in the September 27, 2004 issue of Applied Physics Letters.