Noncontact manipulation of microflow by photothermal control of viscous force

In this paper, we investigate a potential of local control of the viscous force in a microfluidic device for a noncontact microflow manipulation method. Photothermal effect and temperature dependence of the liquid viscosity play a key role to induce an inhomogeneous viscosity distribution in the flow field in a microchannel. Absorption of focused laser beam generates the local change in the viscosity of liquid corresponding to the temperature change. The velocity and temperature fields are measured by the micron-resolution particle image velocimetry and laser-induced fluorescence, respectively. Measurement results indicate that the local reduction of the fluid viscosity due to the temperature rise can cause the change of the flow structure in the microchannel. At the focused area of heating laser beam, namely high temperature area, the flow velocity was increased. The accompanying fluid behavior around the heated region was also recognized. In addition, the agreement between the experimental results and numerical simulation clarifies that the primary factor for the change of the microflow structure is the locally controlled viscous force.