Fluid convection driven by suspended particles in optical trapping

Abstract

We numerically investigate the convection of surrounding fluid in optical trapping of micro- and nanoparticles. The effects of the laser irradiation on the fluid simulation are twofold. First, we take into account the temperature increase of the fluid due the photothermal effect of the solvent, that is, the fluid flow is described by the Navier-Stokes equations under the Boussinesq approximation. Second, we assume that the suspended particles drag the fluid when they are transported by the optical force. This dragging effect is considered in the fluid simulation by adding to the Navier-Stokes equation an external forcing term, which is modelled by considering the counterbalance between the optical scattering force and the Stokes drag. It is shown that the latter effect is dominant under the usual experimental setup in optical trapping of particles with the diameter larger than 0.5 μm. Furthermore, the particle size dependence on the convective flow speed is investigated. The numerical results are supported by optical trapping experiment qualitatively.