In this paper, we report a novel application of the optical trapping of tracers (e.g., micro- or nanoscale particles) to investigate experimentally a microscale fluid flow induced around a microparticle. In particular, we focus on the thermoosmotic slip flows around a target microparticle fixed on a microchannel. The thermo-osmotic slip flows are creeping fluid flows near a solid surface induced by a temperature gradient of the fluid along the surfaces. By confining optically the tracer motions in a circular path around the circumference of a target microparticle immersed in a fluid with an inhomogeneous temperature field, we successfully visualize the flows induced around the target particle along its surface in a direction from the cold to the hot side. It is found that the magnitude of the observed creeping flow depends on the zeta potential of the target microparticles, i.e., the surface charge.
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.
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