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

Abstract

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.

One-dimensional simulation of co-current, dairy spray drying systems – pros and cons

“…Droplet drying using acoustic levitation: In this method, single droplets are suspended using strong acoustic fields in an acoustic levitator. Changes in the droplet's temperature and diameter are measured using infrared thermal devices and digital camera/recorders, respectively [32,42,51,85,86,92,103,[105][106][107][108]. Moisture content profiles are estimated either from diameter profiles or often using a hygrometer to measure the outlet air humidity from the closed levitator.…”

El proceso de secado por atomización: formación de gránulos y cinética de secado de gotas

“…A partir de entonces, empezaron a surgir trabajos de secado de gotas formadas por mezclas binarias de líquidos (agua-metanol y agua-etanol) y suspensiones de micropartículas de vidrio en agua, en los que se establecen las técnicas para la determinación de la porosidad de los gránulos, los periodos de secado existentes y su duración y las tasas de evaporación correspondientes a cada uno de los periodos (5,(34)(35)(36)(37)(38). Todos los estudios mencionados fueron llevados a cabo a temperatura ambiente en su mayoría, debido a la limitación de temperatura que presenta el transductor piezoeléctrico, el cual no soporta más de 60 ºC de temperatura.…”

Microstructure and mechanical properties of grains of silica nanofluids dried in an acoustic levitator

“…This equipment was previously used to study the drying behaviour of single droplets of different materials under a variety of experimental conditions [21][22][23][24][25][26][27][28]. Levitator tubes offer some advantages over conventional methods, since there is no mechanical contact with the droplet and this makes it possible to work with almost any liquid over a wide range of diameters of droplets (from 0.1 mm to several mm) [29].…”