Laser-actuated microfluidic building blocks

Laser induced heating at the interface between two immiscible fluids is used to produce thermocapillary stresses along this interface. When the interface is heated locally, the surface tension is reduced at the hot spot and the fluids are drawn in the direction opposite to the temperature gradient. This effect, known as the Marangoni effect, is amplified in miniaturized systems since the temperature and surface tension gradients are increased as the typical distances are reduced. When implemented in an adequate microchannel geometry, the Marangoni effect allows us to devise fundamental building blocks for microfluidic systems. In particular, the motion along the surface of a fluid immediately causes movement in the bulk for low Reynolds numbers, another feature of miniaturized systems. This allows us to apply the laser heating technique to make a pump in a microchannel, by focusing a laser beam on an oil-water interface. More surprisingly, localized heating can also be applied to create a microfluidic valve when implemented in an adequate geometry. A mixer can also be produced by using a fluid-fluid interface which changes in time. In this talk we present some of our realizations of these actuators in microfluidic channels and discuss some of the physical background underlying their behavior. Once these devices are created they can be combined in a complex circuit, yielding a contactless, scalable solution for the practical limitations that plague microfluidics.