Ferrohydrodynamics governed evaporation phenomenology of sessile droplets

In this article, we report the morphing of the evaporation kinetics of paramagnetic saline sessile droplets in the presence of a magnetic field stimulus. We explore the evaporation kinetics both experimentally and theoretically and study the kinetics on hydrophilic and superhydrophobic substrates for various magnetic field strengths. We show that the evaporation rates of the paramagnetic droplets are augmented significantly and are observed to be a direct function of the magnetic field strength. Additionally, we note the modulation of the contact line transients due to the presence of the field. The influential role of solvated ions in modulating the flow behavior, and subsequently the evaporation, of droplets is present in the literature. Taking cue, we show using particle image velocimetry and infrared thermography that the magnetic field augments the thermo-solutal advection within the droplets. A mathematical analysis, based on the different internal advection mechanisms, has been proposed. We reveal that the magneto-thermal and magneto-solutal modes of internal ferrohydrodynamics are the dominant mechanisms behind the augmented evaporation dynamics. The experimentally obtained internal velocities are in excellent compliance with the model predictions. Furthermore, the enhanced evaporation rates are predicted accurately using a proposed model to scale the interfacial shear modified Stefan flow. The inferences drawn from these findings may hold several important implications in magnetic field-modulated microfluidic thermal and species transport systems.