Experimental study of twin opposing microvortex flows in an opto-electrokinetic microfluidic platform using micronresolution particle image velocimetry technique

A novel technique for the microvortex flow in an opto-electrokinetic microfluidic platform is presented here. A moderately focused laser beam on a pair of indium tin oxide (ITO) electrodes produced two microvortices. This is a novel flow phenomenon consisting of twin opposing microvortices (TOMVs). The vortices contain a fast and strong jet flow region between them. The TOMV flow can be controlled and manipulated by non-uniform electric fields and optical illumination. The non-uniform electric fields were generated by a pair of ITO electrodes partially covered with a SU-8 film. The voltage and frequency of the applied AC signal was varied to study the effect on the flow. Optical illumination, from an infrared laser beam of 1064 nm was controlled by adjusting its power as well as its location relative to the electrodes. In situ generation and control of the TOMV flow allow it to be used as a micro-mixer or pump. The TOMV flow depends on: (1) laser beam location on the electrode, (2) voltage, (3) frequency, and (4) laser beam power. The influence of each parameter was analyzed by micron-resolution particle image velocimetry (µPIV) technique. The location of the laser spot relative to the electrode changed causing the direction and pattern of the TOMV flow, to produce a symmetric, asymmetric or reversed flow, depending on the beam\u27s location. The strength of the TOMV flow was linearly dependent on the voltage in the range between 3–9 Vp-p and laser power in the range between 0.2 to 1.0 W. Frequencies ranging from 3 kHz to 1 MHz can strongly influence the flow strength, in which its dependence follows the logarithmic-Gaussian curve showing the strongest flow at ∼ 107 kHz. The velocities along a cross-section as well as vortex trajectories were analyzed. The experiments presented herein provide practical guidance into applications of microfluidic manipulation in the field of biofluids and biological assay. The previously described TOMV flows were generated mostly in suspensions of 1µm fluorescent polystyrene particles with a single laser beam. In order to investigate the underlying mechanism of the TOMV flow and its feasibility for manipulating various types of suspensions, suspensions of 2µm particles as well as milk emulsions were also used. The superposition of two TOMV flows was demonstrated with two laser beams. Our results demonstrate that this opto-electrokinetic technique has great potential for dynamically manipulating micro-fluid flows