In this paper, we propose a simple model for describing an axis-symmetric thermal convection in a micro channel caused by a photothermal effect, namely, a temperature increase of the fluid due to a laser irradiation. The model consists of two planer solid parts (a microchannel), a thin planer fluid film between the solids, and a focused laser irradiated perpendicularly to the fluid film as a heat source; this is a typical geometrical setting found in various optical trapping experiments. The model describes the flow field and the solid and liquid temperatures. Assuming that the nonlinear convection terms are negligible due to the microscale confinement, the present fluid model is analyzed by two methods: one is a semi-analytical approach and the other is the direct numerical simulation. The validity of the both methods are shown by comparing the results of them, and a typical example of laser-induced thermal convection is presented. The semi-analytical approach is instant and therefore useful even for researchers without the background of fluid mechanics and can be used for systematic prediction of the photothermal fluid phenomena.
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.
Micro- and nanoparticles in a solution under the irradiation of an optical vortex are considered using a mathematical model based on fluid mechanics. The particles exhibit an inherent Brownian motion due to their small sizes. In particular, we consider the case of plural particles trapped in the orbit of the optical vortex expressed by the Laguerre-Gaussian beam. The inter-particle interaction includes not only repulsive forces between the particles but also the forces arising from a hydrodynamic effect. To be more specific, the flow of a solvent induced by the motion of a particle affects the motion of the other particles. The numerical simulation of the model shows that the orbital speed of the particles increases as the number of particles in the orbit.
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