An Experimental Investigation of Unimodal and Bimodal Magnetorheological Fluids with an Application in Prosthetic Devices

The study investigates the field-induced shear yield stress and the off-state viscosity of selected unimodal and bimodal magnetorheological (MR) fluids. Five grades of commercially available iron powder are used to prepare unimodal MR fluids, one for each powder grade, and bimodal MR fluids, using two grades of micron-sized powder. The iron powder contains particles with a mean diameter ranging from 1 to 8 µm. The solid loading of all fluids is held to a constant value while varying the particle size and the ratio between the coarse and the fine powders. All fluids employ a perfluorinated polyether base fluid whose qualities are described. The bimodal MR fluids are compared to their corresponding unimodal fluids. Results show the bimodal fluids to have a lower off-state viscosity than their corresponding unimodal fluids. An application in prosthetic devices is introduced in which the yielding and the off-state characteristics of the MR fluid are of equal importance. In this application, the shear yield stress determines the rigidness while the off-state viscosity determines the flexibility in the absence of a magnetic field. For this particular application, prominent MR fluids are selected based on the ratio between the on-state yield stress and the off-state viscosity.