Abstract
In this research, the convection dynamics of surrounding fluid in optical trapping of tiny particles is investigated both experimentally and numerically. In the experimental part, a tightly focused laser with the wavelength of 1064 nm is irradiated to polystyrene beads with the diameter of 500 nm or 1 um dispersed in water.. The laser plays two roles: (i) it optically traps the particle at the focal spot and (ii) drives the overall particle motion toward the focal spot. The latter effect occurs away from the region where the laser is irradiated, and reaches several tens of micrometers; thus, this long-range particle transport is not induced by the direct action of the laser, but rather by the surrounding fluid motion. In the numerical part, we try to explain this long-range transport by using the simulation of a simple fluid model that includes optical effects both on the particles and the fluid. The model is based on the Navier-Stokes equations with the Boussinesq approximation and additionally includes two optical effects. One is a force acting on the fluid generated by the optically induced motion of suspended particles and the another is a local heat generated in the fluid by the photothermal effect due to the absorption of the laser. By comparing the experimental and numerical results, it is shown that the buoyancy effect by photothermal heating, which has been considered as the cause of the long-range particle transport, is overwhelmed by the effect of the optically induced particle motion.
