Abstract
The goal of this work is to determine the heat transfer mechanisms for digital microfluidic device through the correlation of Rayleigh-SAW streaming and thermal effect, two concomitant phenomena. Péclet number for heat transfer PeTH, which depends on the internal streaming velocity within the droplet, has been used to determine the preponderant transfer mechanism(s). In this sense, determination of flow velocity by Particle Image Velocimetry method and IR measurements, have been necessary. The substrate used was LiNbO3 128Y-X to generate pure Rayleigh SAW. Typically, streaming velocity can reach 40mm.s-1 for 10μl water droplet and PIDT=0.63W, corresponding to PeTH>>1. Forced convection is the predominant mechanism within the water droplet, so the thermodynamic equilibrium is achieved quickly compared to conductive time scale. When the liquid viscosity increases, the streaming is less intensive because of viscous friction and the time constant to reach equilibrium can increase drastically.
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