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Trapping and manipulation of microparticles using Rayleigh convection generated by laser-induced heating of an absorbing thin film

We show that trapping and manipulation of microparticles can be achieved by Rayleigh convection currents using a low power lasers. Light absorbed by thin film of amorphous silicon (a:Si) creates the convection currents. In contrast to previous works, we show that multiple trapping can be achieved using solid microparticles without the creation of vapor bubbles. For low power (~1 mW), particles are trapped at the center of the beam, however at higher powers (~3 mW) particles are trapped on a ring around the beam due to two competing forces: Stokes and thermophoretic forces. Numerical simulations confirm that thermal gradients are responsible for the trapping mechanism.