Field induced deformation of sessile ferrofluid droplets: Effect of particle size distribution on magnetowetting

The field tunable thermal, rheological and optical properties of ferrofluids have been studied extensively and have been exploited in numerous technological applications. However, there are very few studies on the field assisted wetting behavior of ferrofluids, though it is important from fundamental and practical point of view. In this study, we probe the effect of magnetic field induced aggregation on 'magnetowetting' in three different well characterized oil based ferrofluids of similar crystallite sizes (similar to 10 nm) but with different particle size distributions and polydispersity index, through contact angle measurements of sessile droplets. When exposed to a uniform external magnetic field, ferrofluid droplets of all three samples showed elongation along the magnetic field direction, adopting a prolate ellipsoidal meniscus in an effort to balance magnetic force and surface tension. However, the extent of deformation depends on the aggregation kinetics of ferrofluid system as the field induced aggregates are responsible for the meniscus deformation. Further, our studies indicate that at low field strengths, droplet deformation is not significant for systems that displayed rapid aggregation with micron sized thick and long columnar aggregates. The decrease in base fluid-aggregate interaction, brought about by rapid aggregation initiated by significant quantities of larger sized particles, is believed to be the cause for the decreased deformation of meniscus. For magnetic fluids with polydispersity index (PDI) values of 0.79, 0.23 and 0.22, the zero field contact angles were 59 +/- 1.5, 63 +/- 1.3 and 64 +/- 1.2 degrees, respectively. On increasing field strength from 0 to 160 G, the sample with the highest PDI showed a marginal decrease in contact angle from 59 to 54 degrees where the system with minimum PDI showed a significant contact angle change from 64 to 44 degrees. These results will be useful in better control on tunable 'magnetowetting' for practical applications.