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A normal magnetic field has a destabilizing influence on a flat interface between a magnetizable and a non-magnetic fluid. Stabilizing influences are provided by interfacial tension and gravity if the lighter fluid is uppermost. The critical level of magnetization for onset of the instability is derived for a fluid having a non-linear relation between magnetization and magnetic induction. Experiments using a magnetizable fluid, which contains a colloidal suspension of ferromagnetic particles, at interfaces with air and water are made and cover a wide range of density differences. Measurements confirm the prediction for critical magnetization, and it was found that, after onset, the interface took a new form in which the elevation had a regular hexagonal pattern. The pattern was highly stable, and the measured spacing of peaks agreed reasonably with that derived from the critical wave-number for the instability of a flat interface.

Glassy behavior (including hysteresis, irreversibility, a peak in the zero-field-cooled magnetization, and nonexponential relaxation) is observed in a quenched ferrofluid system consisting of 50-A magnetite particles. An

Ferrohydrodynamics treats the flow and thermodynamics of magnetically polarizable fluid in response to applied magnetic field. Following its introduction twenty years ago, ferro-hydrodynamics has developed as a branch of mechanics similar to magnetohydrodynamics and electrohydrodynamics. Unique phenomena of ferrofluids and related media are manifested in hydrostatics, inviscid and viscous flows, magnetically induced flow instability, magnetic stabilization of flow, generation of antisymmetric stress, and magnetic multiphase flow. Certain of the phenomena serve as bases of developed technology in seals, dampers, accelerometers, and cooling techniques with new applications under active development in tribology, acoustics, printing, instrumentation, and other areas. Ferrohydrodynamic principles underlie also the development of magnetic multiphase systems yielding turbulence prevention of moving-bed operations in the process industry. Major concepts and advances of the field are presented in the context of the continuum mechanical framework.

A phenomenological treatment is given for the fluid dynamics and thermodynamics of strongly polarizable magnetic fluid continua in the presence of nonuniform magnetic fields. Examples of the fluids treated here have only recently been synthesized in the laboratory. It is found that vorticity may be generated by thermomagnetic interaction even in the absence of viscosity and this leads to the development of augmented Bernoulli relationships. An illustration of a free-surface problem of static equilibrium is confirmed by experiment and information is obtained regarding a fluid's magnetic susceptibility. Another illustration elucidates the mechanism of an energy conversion technique. Finally, an analytical solution is found for the problem of source flow with heat addition in order to display the thermomagnetic and magnetomechanical effects attendant to simultaneous heat addition and fluid motion in the presence of a magnetic field.

Oleic acid and stearic acid are similar surfactants which, however, lead respectively to stability and to precipitation of ferrofluid suspensions: to understand this, the forces between layers of oleic-like surfactants and between layers of stearic-like surfactants across a hexadecane (HD) medium were measured using a surface force balance (SFB). Separate measurements reveal that only the oleic layers are solvated by HD, while the SFB results reveal that for both surfactants a marked net attraction is present between the surfaces. Simple considerations based on these observations explain why, despite this attraction, ferrofluid dispersions are stabilized by oleic but not by stearic surfactants.

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