Drying of acoustically levitated droplets of liquid-solid suspensions: Evaporation and crust formation

Measurements on the drying behavior of droplets of aqueous suspensions of glass beads are carried out in a tube levitator at varying drying conditions. The experimental results show that there exist two distinct drying stages. At stage I the droplet diameter decreases. At stage II it is constant, while solvent evaporation still proceeds, as it follows from the permanent upward motion of the drop toward the pressure node. A theory for describing the evaporation of a liquid/solid suspension droplet in the acoustic field at the first drying stage is developed and validated with the measurement data. The theory solves the mass transfer problem of liquid vapor from the droplet surface. It takes the convective influence of the acoustic field on the mass transfer into account, using results from the earlier work of the authors on the evaporation of acoustically levitated droplets. Comparison of the theory with the experiments supports the idea that at the first drying stage solvent evaporation proceeds from the wetted surface of suspension droplets. The acoustically driven mechanism of the evaporation process is related to the acoustic streaming flow. The present work does not propose any theory for the second drying stage. It presents only some physical estimates, suggesting that stage II begins from a relatively fast precipitation of the suspended particles (glass beads) in the outer layers of the droplet, and crust formation. The estimates also suggest that the further solvent evaporation proceeds through the pores of the crust. Liquid evaporation in the pores, according to the estimates, proceeds according to the ordinary diffusion-driven mechanism, which is a limiting process in this case.