Laser-controlled microscale fluid flows near the air-liquid interface of suspension droplets

We investigate microscale flows of microparticles driven by a laser beam focused near the air-liquid interface of a suspension droplet. Three distinct regimes, convection, a linear flow, and a nonflow, are found by controlling both the position of the beam focus relative to the air-liquid interface and the laser power. They are governed by the most dominant of two effects exerted by a focused laser beam, i.e., local heating and radiation pressure. We find that in the nonflow regime two-dimensional close-packed arrays of microparticles are formed on the air-liquid interface, and spin on the beam axis. We show that micron-sized polystyrene beads can be bonded into a long chain structure by taking advantage of the linear flow.