Morphing evaporative heat and mass transport of nanofluid droplets by electric field

Abstract

In this research, evaporation behaviour of pendant droplets of stable nanocolloidal dispersions in the presence of an electric field is probed, both experimentally and theoretically. It is observed that the colloid droplets evaporate faster than their water counterparts in absence of the field. But within field environment, increase of electric field strength suppresses the evaporation rates, while the field frequency shows no appreciable effects on the evaporation rates. Also, the reduction of evaporation rate in field environment is pronounced at higher colloidal concentrations. Theoretical analyses from existing models reveal that neither alterations in the surface tension nor the diffusion-driven classical evaporation model can map the reduced evaporation rates. Through infrared thermography and Particle Image Velocimetry, reduction of internal thermo-solutal circulation velocity for the droplet is noted when stimulated by the electric field, which is shown to directly affect the rate of evaporation. The effects of electrohydrodynamic advection, electro-thermal and electro-solutal convection on the evaporation behaviour is modelled by a scaling approach. The influence of dominant non-dimensional numbers, such as thermal Marangoni number, solutal Marangoni number, electro-Prandtl number, electro-Schmidt number, and the Electrohydrodynamic number, are quantified and discussed. Stability considerations reveal that the stable internal flow behaviour is retarded by the electric body force, with the reduction via the electro-solutal route being predominant, and the internal flow velocities being mapped well by the electro-solutal model. The findings may hold implications in the domain of multiphase transport phenomena of complex fluids at micro and macroscales.