Electromagnetic field orientation and characteristics governed hydrodynamics within pendent droplets

Abstract

This article reports the dominant governing role played by the direction of electric and magnetic fields on the internal advection pattern and strength within salt solution pendent droplets. The literature shows that solutal advection drives circulation cells within salt based droplets, even in the absence of any external field. An experimental study is performed, where electric and magnetic fields are applied across pendent droplets of salt solutions and their internal flow dynamics is observed. Flow visualization and velocimetry (two-dimensional) reveals that the direction of the applied field governs the enhancement or reduction in circulation velocity and the directionality of circulation inside the droplet. Further, it is noted that while magnetic fields augment the circulation velocity (with respect to the solutal advection already present in salt solution droplets at zero field) the electric field leads to deterioration of the same. The concepts of electro- and magnetohydrodynamics of droplets are appealed to and a Stokesian stream function based mathematical model to deduce the field mediated velocities has been proposed. The roles of the governing Hartmann, Stuart, Reynolds, and Masuda numbers are revealed by the proposed model. The theoretical predictions are observed to be in agreement with the experimentally determined averaged spatiotemporal circulation velocities. The present findings may have strong implications in microscale and interfacial electro- and/or magnetohydrodynamics.