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.
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