Ring stains in the presence of electromagnetohydrodynamic interactions

In a recent paper [Das et al., Phys. Rev. E 85, 046311 (2012)], we delineated the role of electrokinetic transport in modifying the classical "coffee stain" effect. In this study, we extend this calculation to incorporate the consequences of a generalized electromagnetohydrodynamic transport in the coffee stain phenomenon. The magnetohydrodynamic (MHD) effect enhances the velocities at the beginning of the drop life, whereas the electrokinetic effect increases the "disordering" effect in particle deposition at the end of the drop, triggered by a velocity divergence. For a suitable combination of the strength of the MHD and electrokinetic transport, however, this disordering effect is substantially enhanced, and, most nonintuitively, such velocity divergence and the disordering effect may occur at a time that is much earlier than the end of the drop life, or may occur even instantaneously after the start of the drop evaporation. This work will provide useful insight in the understanding of the dynamics of mesoscopic patterns formed as the magnetic nanocrystals deposit in the presence of a combined transport driven by evaporation and magnetic field effects.