Using micro particle image velocimetry (μPIV) [1], we visualize and analyze strong toroidal microfluidic vortices generated when a near-infrared (1064 nm) laser beam and an alternating current (AC) electric field is simultaneously applied to a chip. The vortices exhibit a source type behavior in the plane normal to the electric field and the flow vorticity is characterized as a function of the electric field potential, electrical AC frequency and laser power. At a constant AC frequency of 9 kHz, the flow vorticity increase as the square of the electric field strength. At constant electric field, the flow vorticity does not change appreciably in 10–100 kHz range and it decreases at larger frequencies (>500 kHz) until when Brownian motion dominates the movement of the 3μm tracer particles. At constant electrical frequency and voltage of 9 kHz and 20Vpp, the flow vorticity remarkably increases as the laser power increases from 20 to 100mW, due to the rapid change of the temperature gradient inside a fluid and its coupling effect with the applied electric field.

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