Magnetofluidic mixing of a ferrofluid droplet under the influence of a time-dependent external field

We report experimental investigations on the mixing of a ferrofluid droplet with a non-magnetic miscible fluid in the presence of a time-dependent magnetic field on an open surface microfluidic platform. The bright-field visualization technique, in combination with micro-particle image velocimetry analysis, is carried out to explore the internal hydrodynamics of the ferrofluid droplet. Also, using the laser-induced fluorescence technique, we quantify the mass transfer occurring between the two droplets, which in effect, determines the underlying mixing performance under the modulation of the frequency of the applied magnetic field. We show that the magnetic nanoparticles exhibit complex spatio-temporal movements inside the ferrofluid droplet domain in a transient magnetic forcing environment, which, in turn, promotes the mixing efficiency in the convective mixing regime. Our analysis establishes that the movement of magnetic nanoparticles in the presence of the time-periodic field strengthens the flow instability, which initiates an augmented mixing in the present scenario. By performing numerical simulations, we also review the onset of instability phenomena, mainly stemming from the susceptibility mismatch between the magnetic and non-magnetic fluids. Inferences of the present analysis, which focuses on the simple, wireless, robust and low-cost open surface micromixing mechanism, will provide a potential solution for rapid droplet mixing without requiring a pH level or ion concentration dependency of the fluids.