Spin-up flow in ferrofluids: a toy model.

It is now more than 50 years since the first observation of ferrofluid spin-up by a rotating magnetic field by the author and R. Moskowitz. The ferrofluid rotated in direction opposite to that of the applied field. The intervening years have seen multiple unsuccessful attempts to explain this phenomenon. A clue was experimentally discovered by J. Popplewell and the author in 1987; rotation direction depends on shape of the ferrofluid meniscus. That notion is studied in further detail in the present work which describes tangential stress generated on the meniscus surface and an analytical expression for torque is developed. Increasing spin rate in smaller diameter vessels is rationalized. In addition, a physical picture of spin-up is introduced.

Fluidization: hydrodynamic stabilization with a magnetic field.

Fluidization of magnetizable particles by a gas stream in the presence of a uniform applied magnetic field oriented parallel to the flow prevents the hydrodynamic instability that otherwise leads to bubbles and turbulent motion within the medium. The fluidized emulsion expands uniformly in response to gas flow speeds in excess of that at the incipient fluidization point, with transition from the quiescent stable state to bubbling occurring suddenly at a characteristic increased rate of flow. Experimental data demonstrate the dependence of this transition velocity on the intensity of the applied magnetic field, length of the bed, and type of magnetic solids. Data illustrate the pressure distribution through the bed medium, the bedflow characteristics, and other phenomena.

Study of audio speakers containing ferrofluid.

This work validates a method for increasing the radial restoring force on the voice coil in audio speakers containing ferrofluid. In addition, a study is made of factors influencing splash loss of the ferrofluid due to shock. Ferrohydrodynamic analysis is employed throughout to model behavior, and predictions are compared to experimental data.