Optically induced, AC electrokinetic manipulation of colloids

This report introduces a novel optically induced AC electrokinetic technique that generates microfluidic motion as well as rapidly and continuously accumulates micro- and nanoparticles on an unmodified planar electrode surface. This method has been termed rapid electrokinetic patterning (REP). A highly focused optical landscape was applied to the surface of a parallel-plate electrode at a wavelength of 1,064 nm and laser powers typically less than 30 mW. The focused illumination produced sharp thermal gradients within a fluid sample injected between the electrodes. An applied electric field acted upon these gradients, resulting in electrothermal hydrodynamic motion. The resulting microfluidic motion resembled a microfluidic vortex, carrying suspended particles towards its center. These particles were trapped on the electrode surface by low-frequency (< 300 kHz) electrokinetic forces. Colloid accumulation resembled a crystalline, monolayer assembly and therefore this technique could be used to create artificial architectures. Particle aggregation was AC frequency dependent, enabling colloidal sorting as well as investigation of low-frequency colloid polarization mechanisms.;The temperature profile from the optically induced heating was measured with temperature dependent fluorescent dye. With an applied laser power of 30 mW the maximum measured heating was 5.5°C. The optically induced electrothermal microfluidic vortex was analyzed with 2D muPIV (Particle Image Velocimetry) techniques. Computer simulations were used to model this vortex and confirm observed experimental trends. The velocity of the microfluidic vortex increased with the square of the applied electric field. Electrothermal fluid velocity remained constant for fluid conductivities and applied AC frequencies less than at its charge relaxation.;The particle aggregation was characterized with various applied AC voltages, AC frequencies, illumination intensities, and fluid conductivities. The frequency dependent nature of particle accumulation was investigated, including utilizing this technique for selective particle accumulation and separation. Silica, polystyrene, and latex particles were captured, ranging from 49 nm to 3.0 microns in diameter. Experiments confirmed the electrokinetic mechanisms involved with particle capture, including frequency dependent polarization mechanisms and particle-particle interactions. Results provide a foundation for future investigations to apply this dynamic particle and fluid manipulation technique.