Optical manipulation of droplets in microfluidics

Abstract

In this paper, we investigate optical particle separation method for separating the micro emulsions by their refractive indices. Micro emulsions of two different chemical species are generated with T-junction method with the controlled size and frequency by the flow rate. Heavy water emulsions were generated using the fluorinated oil as continuous phase as water in oil emulsion scheme. The size and frequency of the emulsions can be determined by the flow rate of the disperse phase and continuous phase. The particles are injected and flowed following the microchannel and pushed to normal to the flow direction by the optical force from a continuous laser in the light field area. Because the optical forces exerting on the emulsions are different from its refractive index and size, each transported chemical species emulsions have different retention distance in the continuous manner and can be separated simultaneously. The retention distance defined as particle’s displacement perpendicular to flow direction in microfluidic channel while it is deflected by optical force perpendicular to flow direction. To enhance the system performance, T-junction and bifurcation junc-tion were designed to align the droplets intended direction. Navier-stokes equation with large particle was solved with commercial software. To generate and manipulate droplets, micro channel was designed, fabricated, and chemically treated. The optical forces exerting on droplets and their trajecto-ries were calculated and evaluated with experiment data. The retention distance of droplets assessed with experiment data and numerical calculation. Because large droplet and channel dimension ratio, drag force is much higher than small particle at the same condition. The behavior of the particle in tightly confined micro channel was derived and compared. Therefore, separating by its retention distance within micro fluidic channel was achieved. Also, droplet trapping and coalescence were demon...