Ferrofluids are suspensions of magnetic nanoparticles that have the attractive feature of being controlled by applied magnetic fields. Ferrofluids have been studied for decades in an ever growing number of applications that take advantage of their response to applied magnetic fields. Here, we provide a summary of recent advances in established and emerging applications of ferrofluids, including applications in optics, sensors, actuators, seals, lubrication, and static/dynamic magnetically driven assembly of structures.

Ferrofluid flow in cylindrical and annular geometries under the influence of a uniform rotating magnetic field was studied experimentally using aqueous ferrofluids consisting of low concentrations (<0.01 v/v) of cobalt ferrite nanoparticles with Brownian relaxation to test the ferrohydrodynamic equations, elucidate the existence of couple stresses, and determine the value of the spin viscosity in these fluids. An ultrasound technique was used to measure bulk velocity profiles in the spin-up (cylindrical) and annular geometries, varying the intensity and frequency of the rotating magnetic field generated by a two pole stator winding. Additionally, torque measurements in the cylindrical geometry were made. Results show rigid-body like velocity profiles in the bulk, and no dependence on the axial direction. Experimental velocity profiles were in quantitative agreement with the predictions of the spin diffusion theory, with a value of the spin viscosity of ∼10⁻⁸ kg m/s, two orders of magnitude larger than the value estimated earlier for iron oxide based ferrofluids, and 12 orders of magnitude larger than estimated using dimensional arguments valid in the infinite dilution limit. These results provide further evidence of the existence of couple stresses in ferrofluids and their role in driving the spin-up flow phenomenon.

Computer simulations are reported for hard superellipse particle phases vs. aspect ratio and curvature. Findings indicate design rules for how shape determines diverse two-dimensional liquid, liquid crystalline, and crystalline microstructures.

An asymptotic solution is obtained for flow of a ferrofluid between two stationary coaxial cylinders of infinite extent, including the effects of spin viscosity. This solution takes into account the nonuniformity of the magnetic field in the annular space that arises as a consequence of the demagnetizing field of the inner cylinder. In the limit of zero spin viscosity the analysis predicts there is no flow in the annular gap. For nonzero spin viscosity the analysis predicts flow in the annulus with corotation of field and fluid close to the outer cylinder wall and counter-rotation close to the inner cylinder wall. The asymptotic predictions for the translational velocity are compared to experimental measurements obtained using the ultrasound velocity profile method for ferrofluid in an annular gap. The observed experimental velocity profiles are in qualitative agreement with the predictions of the asymptotic model, and transition between corotation to counter-rotation of fluid and field was observed at an intermediate radial position. These observations provide further evidence of the existence of couple stresses in ferrofluids and the importance of spin viscosity in describing some ferrofluid flows.