Coupled Heat and Mass Transport in Liquid Films That Contain Thermophoretically Active Particles

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We consider the spontaneous redistribution of thermophoretically active particles suspended in a thin film of liquid, made to absorb energy and transmit it in the form of heat to the surrounding medium. When the opposing boundaries to the thin dimension are maintained at a constant temperature, a nonuniform temperature profile is formed across the film because of differential heat dissipation, which is maximized at the boundaries. Thermophobic particles move and concentrate at the opposing (cooler) boundaries, whereas thermophilic particles concentrate within a layer midway between the boundaries. The Dufour-like effect results in a synergism between concentration and temperature profiles that enhances the temperature gradient. Increases in particle concentration up to 10-fold can be achieved rapidly for non-Janus particles at room temperature using low energy input to maintain transverse temperature differences of a few degrees Kelvin. This level of concentration is much higher than that predicted for Janus particles, where mass diffusion coefficients are larger and less dependent on temperature. Finally, in contrast to Janus particles, the system is expected to remain stable with both positive and negative thermophoresis.