Abstract
Photothermal effect is energy conversion process from light to heat due to absorption. Using a focused laser beam, the photothermal effect of working fluid can be used to produce a localized heat spot in microfluidic devices. The locally-heated fluid induces both thermal convection and thermophoresis: The former is the motion of fluid due to a buoyancy force and the latter is the motion of dispersed particles along the temperature gradient of the fluid. In this research, the competition between these two effects is experimentally investigated by using a microgap fluidic channel with a variable gap size. Thermal convection is more affected by the channel confinement and becomes weaker as the gap size decreases. Therefore, the thermal-convection-dominant state is observed when the gap size is above a threshold. However, when the gap size is below the threshold, the thermophoresis-dominant-state appears. This experiment demonstrates a switching method between these two competing states by simply changing a channel dimension.
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