Droplet-based microfluidics which involves discrete volumes with the use of immiscible phases enable controlled and rapid mixing inside the droplet and promoted reaction of reagents or cells. It can be operated as "digital fluidic platform." Due to high surface area to volume ratio of transport phenomena in microscale, an interfacial behavior becomes more predominant than continuous-flow-based microfluidics. In this study, we have investigated an interfacial flow control based on local photothermal excitation of the interfacial tension gradient resulting in Marangoni convection for droplet manipulation in a microfluidic chip. The surface Marangoni flow occurs by the local thermal gradient induced by the localized light irradiation which is spatially characterized by a mask with a specific aperture geometry. In controlled droplet generation and manipulation, oil-in-water (O/W) system, oleic acid as the dispersed phase, were used in the present experiments. Droplets have volumes from 0.5 to 65 pL, corresponding to diameters from 10 to 50 μm. A microfluidic chip consists of two PDMS (polydimethylsiloxiane) channel layers fabricated using the softlithography. Spatially characterized heating is produced by a DPSS laser with a wavelength of 532 nanometers, a mask with aperture and a reduced-projection exposure optics. The light irradiation generates local temperature change in the continuous phase which can cause interfacial tension gradient when droplets come to the illuminated area. As a result, the droplet experiences a repulsion force from the illuminated area with high temperature because the liquid-liquid interface in this case has positive temperature dependence on the tension. The droplet can be trapped in the microchannel when U- or V- shaped light pattern is irradiated. When a light pattern with nozzle-like geometry is irradiated, droplets were focused toward the exit of the nozzle avoiding the irradiated area. The performances of the trapping and focusing of droplets due to the optically-induced interfacial flow were evaluated through behaviors of droplets with different sizes and light powers. The estimation of forces acting on a drop due to the photothermal Marangoni convection was also conducted.
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