Optothermorheological flow manipulation.

Optical methods for microfluidic flow manipulation offer a flexible, noncontact technique for both fluid actuation and valving. At present, however, such techniques are limited by their high laser power requirements, low achieved flow rates, or poor valve switching times. Here we demonstrate a microfluidic valving technique based on optothermorheological manipulation using a low-power 40 mW laser with switching times on the order of 1 s at high flow rates of 1 mm/s. In our approach a laser beam incident on an absorbing substrate is used to locally heat a thermorheological fluid flowing in a microfluidic channel. The resulting gelation in the heated region creates a reversible fluid valve.