Microflow Behavior of Liquid in the Presence of Laser-Induced Temperature Gradient

A novel technique is presented for non-intrusive microflow control with laser-induced local temperature gradient. In microscale, fluid behavior is quite different from that in macroscale, especially an effect of fluid or interfacial properties on microflow becomes significant. The technique described in this paper utilizes the property change of fluids caused by a light-induced temperature change, i.e., photothermal effect. Absorption of a focused laser beam causes the local spot in temperature. Since the viscosity of general fluids has strong temperature dependence, a generation of local temperature gradient in microfluids results in the corresponding viscosity distribution, which is directly related to the flow behavior in microflow. In order to demonstrate the validity of this concept, we have developed an experimental system to irradiate focused laser beam on a flow in a microfluidic device and to measure velocity profile of the microflow simultaneously. As a heating source, a compact laser diode (LD) with the visible wavelength of 635nm is employed. Velocity measurement is performed by a micro particle image velocimetry (micro-PIV) technique. Optical separation between LD absorption and excitation/emission of fluorescent particle for micro-PIV measurement is confirmed. Change in the microflow behavior in a rectangular microchannel (500 μm × 50 μm) during the LD irradiation due to the photothermal effect is observed.