In Situ Change in Contact Angle of a Sessile Droplet on a Magnetoactive Elastomer

ABSTRACT Conventional characterization of magnetically tunable wetting properties on magnetoactive elastomer (MAE) surfaces prompts for the re‐deposition of water droplets when the surface state is to be changed, e.g., via the magnetic field. Motivated by potential applications of these smart materials in digital microfluidics, we experimentally investigate the wetting of MAE surfaces in time‐varying magnetic fields. Not only is the dynamical contact angle considered, but also the externally applied magnetic field can vary with time (square‐wave, rising‐edge and falling‐edge step excitations). It is found that the wetting characteristics of water droplets permanently present on MAE surfaces are different from the conventional approach, i.e., an increase in the magnetic field does not normally lead to an increase in the contact angle (CA). Additionally, it is hypothesized that formation and magnetic‐field enhancement of the wetting ridge on the surface of a soft (effective shear modulus 10 kPa) MAE play an important role in dynamic wetting. A simple, purely geometrical theory is capable of describing the initial growth of the CA of an inflating droplet with time for the rising‐edge step magnetic field. Observed physical effects are discussed in the view of modern concepts regarding the surface and bulk properties of MAEs.