  The present article discusses the physics and mechanics of evaporation of
pendent, aqueous ferrofluid droplets and modulation of the same by external
magnetic field. We show experimentally and by mathematical analysis that the
presence of magnetic field improves the evaporation rates of ferrofluid
droplets. First we tackle the question of improved evaporation of the colloidal
droplets compared to water, and propose physical mechanisms to explain the
same. Experiments show that the changes in evaporation rates aided by the
magnetic field cannot be explained on the basis of changes in surface tension,
or based on classical diffusion driven evaporation models. Probing using
particle image velocimetry shows that the internal advection kinetics of such
droplets plays a direct role towards the augmented evaporation rates by
modulating the associated Stefan flow. Infrared thermography reveal changes in
the thermal gradients within the droplet and evaluating the dynamic surface
tension reveals presence of solutal gradients within the droplet, both brought
about by the external field. Based on the premise, a scaling analysis of the
internal magnetothermal and magnetosolutal ferroadvection behavior is
presented. The model incorporates the role of the governing Hartmann number,
the magnetothermal Prandtl number and the magnetosolutal Schmidt number. The
analysis and stability maps reveal that the magneto-solutal ferroadvection is
the more dominant mechanism, and the model is able to predict the internal
advection velocities with accuracy. Further, another scaling model to predict
the modified Stefan flow is proposed, and is found to accurately predict the
improved evaporation rates.
