Ronald E. Rosensweig

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.

Fluid Dynamics and Science of Magnetic Liquids

Publisher Summary This chapter discusses the fluid dynamics and science of magnetic liquids. Fluid media composed of solid magnetic particles of subdomain size colloidally dispersed in a liquid carrier are the basis for the highly stable, strongly magnetizable liquids known as magnetic fluids or ferrofluids. The number density of particles in suspension is in the order of lO 23 /m 3 . It is the existence of these synthetic materials that makes the study of magnetic liquid fluid dynamics (ferrohydrodynamics) possible. There are two broad ways to make a magnetic fluid: size reduction of coarse material and chemical precipitation of small particles. Size reduction has been done by spark evaporation–condensation, electrolysis, and grinding. Chemical routes include decomposition of metal carbonyls and precipitation from salt solutions. The fluid dynamics of magnetic fluids differ from that of ordinary fluids in that stresses of magnetic origin appear and, unlike in magnetohydrodynamics, there need not be electrical currents. Magnetic fluid pushes the nonmagnetic fluid in the presence of tangential applied field. The fluid motion is normal to the interfacial boundary between the two fluids.

Labyrinthine instability in magnetic and dielectric fluids

Abstract The magnetic and electric duality of magnetizable and polarizable liquids is demonstrated for the labyrinthine instability that occurs in thin layers subjected to initially uniform fields. Theory is developed for the spacing of fully developed labyrinth patterns in horizontal cells, and measurements compared with theory.

Magnetic fluid motion in rotating field

Abstract Through experiment and analysis, tangential free-surface-stress is identified as the dominant mechanism in the coupling of uniform rotary magnetic fields to the spin-up motion of colloidal magnetic fluid.

“Negative Viscosity” in a Magnetic Fluid

Fluids containing colloidal dispersions of magnetic particles exhibit new phenomena, including a reduction in viscosity under some conditions. In his Perspective, Rosensweig reviews the theoretical and experimental state of magnetic fluid studies.

Magnetorheology of ferrofluid composites

Composites consisting of nonmagnetic particles with sizes in the micron range suspended in a ferrofluid constitute an inverse magnetorheological fluid. Structuring occurs in an applied magnetic field and can result in the solidification of the composite. Above a critical level of applied stress the material further transforms to a liquid state. Data confirming the existence of a solidified state are presented based on constant shear rheological measurements. Measurements of the yield stress compare favorably with predictions of theory based on the analysis of unsymmetric stresses in the unyielded, anisotropic medium.

Excitation and study of subcritical waves on a magnetic fluid surface

Abstract We find that subcritical standing waves are excited when a time-periodic field is superposed collinearly on a steady field with normal orientation to a pool of magnetic fluid. The wavelengths of the disturbance undergo repeated, stepwise reductions as the steady field intensity is increased. A theory of the onset is derived and comparison is made with the data.

Magnetic field induced rotations in ferrofluids

An experimental study has been made of the fluid rotations observed at the surface of a water-based ferrofluid ( mu /sub 0/M/sub s/=0.006 T) when the fluid is in a rotating magnetic field with values up to 0.03 T and a frequency of 50 to 500 Hz. It is shown that the fluid rotation is surface-driven and that the rotation rate increases with both frequency and field. The direction of rotation is determined by the surface curvature. The study has been extended to include analysis of the rotational behavior of micron-size nonmagnetic particles immersed in ferrofluid in the same rotating field conditions. In this case, the torque at a sufficiently high frequency acting at the particle-ferrofluid interface produces a rotation of particles counter to the field direction. The corotation with field of small particles, >

Transient and Cyclic Behavior of Cellular Automata with Null Boundary Conditions

One-dimensional cellular automata (CA) over finite fields are studied in which each interior cell is updated to contain the sum of the previous values of its two nearest neighbors. Boundary cells are updated according to null boundary conditions. For a given initial configuration, the CA evolves through transient configurations to an attracting cycle. The dependence of the maximal transient length and maximal cycle length on the number of cells is investigated. Both can be determined from the minimal polynomial of the update matrix, which in this case satisfies a useful recurrence relation. With cell values from a field of characteristic two, the explicit dependence of the maximal transient length on the number of cells is determined. Extensions and directions for future work are presented.

Magnetic field gradient effects on magnetic fluid stabilization

Abstract The penetrating finger instability which develops when a less viscous fluid pushes a more viscous fluid can be stabilized through the use of a magnetizable fluid in the presence of a magnetic field tangential to the interface. A uniform magnetic field only stabilizes suitably short waves travelling along the field lines. Transverse waves of all wavelengths and orientations are also stabilized if the tangential magnetic field is non-uniform with field decreasing in the direction away from the magnetically permeable fluid. Confirming experiments are described using laboratory sandpacks.