Mark A. Golden

Mason numbers for magnetorheology

The electric field strength and shear rate dependence of the apparent shear viscosity of electrorheological (ER) suspensions can often be represented by a function of only the Mason number. A Mason number defined for magnetorheological (MR) suspensions by direct substitution of magnetostatic variables for electrostatic variables does not produce a similar collapse of shear viscosity data for MR suspensions. We show that a Mason number defined in terms of the suspension magnetization can be employed to produce a collapse of experimental data at various magnetic field strengths and shear rates. As for ER suspensions, this Mason number can be calculated from experimentally measured quantities.

Enhancing magnetorheology with nonmagnetizable particles

Experimental results illustrate an enhancement in the field-induced yield stress of magnetorheological (MR) fluids caused by the presence of nonmagnetizable particles. Particle-level simulations in three dimensions show similar behavior. However, the enhancement does not appear in simulations in which the spheres are confined to a monolayer. A mechanistic explanation of these observations is currently lacking. Nonetheless, the ability to enhance the MR response by replacing magnetizable particles with nonmagnetizable particles offers several advantages for applications.

Transient response of magnetorheological fluids: Shear flow between concentric cylinders

An experimental investigation of the rheological response of magnetorheological suspensions subjected to step changes in applied magnetic field strength at fixed shear rate is reported. For small applied field strengths, the shear stress increases rapidly to a steady value. Above a critical field strength, the rapid initial increase in shear stress is followed by a slow, transient increase in stress. The critical Mason number corresponding to the critical magnetic field strength at the onset of this transient depends on the particle volume fraction as well as the shear rate. This is in contrast to a previous analysis where the critical Mason number was predicted to depend on only the particle volume fraction. The discrepancy is attributed to colloidal forces that are significant in our experimental system, but were not included in the analysis. Further comparison with the previous analysis requires either including the effects of colloidal forces, or performing experiments with systems in which colloidal ...

Effects of nonmagnetic interparticle forces on magnetorheological fluids

Effects of nonmagnetic interparticle forces on the on- and off-state behavior of magnetorheological fluids are investigated experimentally and with particle-level simulations. Suspensions of iron particles in an aliphatic oil are modified by surface-active species. The modifications significantly alter the off-state properties, but have little impact on the field-induced stresses. Simulations show similar behavior. Off-state rheological properties are strongly influenced by van der Waals forces and modifications of the short-range repulsive forces. Field-induced stresses are less sensitive to the nonmagnetic forces.

EVALUATION OF YIELD STRESS MEASUREMENT TECHNIQUES ON A PARALLEL PLATE MAGNETIC RHEOMETER

The on-state yield stress is the primary design parameter for an MR actuator. We have evaluated several methods for measuring yield stress on a number of MR fluids of varying composition using a commercial parallel plate magnetic rheometer. The data obtained by these methods is evaluated for reproducibility and sensitivity to sample size variation and is compared against data obtained with a concentric cylinder magnetic rheometer and literature results. Based on these evaluations and comparisons, we selected two tests for further evaluation.

The Coefficient of Friction of a Polyamide/Polymethyl Methacrylate Blend System

The coefficient of friction is an important tribological property of polymer materials. In this work, values for the coefficient of friction of a polyamide (nylon 6,6)/polymethyl methacrylate (PMMA) blend system have been measured under various compressive stresses. The results show that the coeffi cient of friction of the amorphous polymer, PMMA, increases with increasing compressive stress; while the coefficient of friction of the crystalline polymer, nylon 6,6, either stays constant or decreases with increasing compressive stress. For crystalline polymers, the coefficient of friction measured parallel to the flow direction is lower than that measured perpendicular to the flow direc tion. The effect of sample orientation on the coefficient of friction of the amor phous polymer, PMMA, is insignificant. Values for the coefficient of friction of the blends measured are lower than those calculated from the additive rule for the coefficient of friction of the blend system. It is noteworthy that values for the coefficient of friction of several specific blend compositions are lower than those of the two thermoplastics. These results are in good agreement with the predictions from the tribological controlling equations developed. Based on these findings, nylon 6,6/PMMA blends with low values of the coefficient of fric tion have been successfully developed.

Modeling of MR Fluids and Devices

Magnetorheological (MR) fluids and MR fluid devices have been under development at General Motors RD an analysis of the transient shear behavior of MR fluids; a model of the behavior of MR particles in a gravitational field; and a lumped parameter model of a MR clutch. For each example, we will discuss the motivation that led to the study, the methods used to try to answer the question that led to the work and conclude with a brief discussion of other potential work that could be done.