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

The microstructure and mechanical properties of spray-dried grains are key factors in many applications. In this work single droplets of silica nanofluids were dried in an acoustic levitator under different experimental conditions of solid mass fraction, pH value, salt concentration, drying temperature and initial droplet volume. ANOVA method was used to determine their influence on the final grain diameter, the shell thickness and the mechanical strength. The solid content and the droplet volume are the variables that exert an influence on these three properties. In addition, the mechanical strength is influenced by the pH value. The maximum packing fraction of the particles inside the shell was obtained by modeling the viscosity data with the Quemada equation. The packing fraction was then used to calculate the shell thickness. The internal microstructure of the grains was observed by SEM and the shell thickness was measured. Experimental and calculated results show good agreement.