Assembly of microscopic highly magnetic droplets: Magnetic alignment versus viscous drag

Abstract

We report here the collective behavior of droplets in the scale ${10}^{-6}–{10}^{-5}\mathrm{m}$ made of a concentrated magnetic fluid elongated and oriented by a magnetic field while rotating synchronously with respect to the carrier liquid. Distribution of droplet sizes is studied as a function of a magnetoviscous number $N_{\mathrm{mv}}$ that quantifies the competition between magnetic field and local vorticity. The liquid state and very low interfacial tension enable both breakup and coalescence processes, which are undergone by the droplet population to reach dynamic equilibrium. Theoretical analysis of a single drop motion is extended to the case of the drop assembly. Experiments combining rotation and field modulation show a regime of nonsteady rotation in good agreement with theory.