Magnetic Field Assisted Evaporation of Droplets On Viscoelastic Surfaces

We investigate the dynamics of droplet evaporation atop substrates of varying elasticity, in presence of homogenous and heterogeneous magnetic fields. PDMS was chosen as the base substrate, with its elasticity varied by changing the base to cross-linker ratio. The substrates were doped with a constant weight of ferrofluid, so as to render them magnetic. Water droplets with volume of 500 nL were used for all the experiments. Two magnets (annulus shaped; magnetized axially) were placed adjacent to each other, with an end to end spacing of 5 mm; with two different configurations viz. both the North-poles (NN) facing the substrate (placed atop the magnet) and North-South (NS); with the droplet dispensed at their midpoint. We evince that, for the case of NN configuration, the resultant field lines at the center were oriented normal (downward) to the substrate, whereas for the NS configuration, they were oriented along the substrate. As a result, there was a substantial increase in the duration of evaporation of the droplet for the case of NS, as compared to that of NN, due to a lateral shift in the translation of the magnetic particles, within the substrate. We conjecture that, the present study could be useful for effective control and manipulation of the droplet.