Squeeze flow and polydispersity effects in magnetorheology

Abstract

Generally speaking, conventional magnetorheological fluids are colloidal suspensions of ferromagnetic particles in a non-magnetic continuous medium. Under the application of an external magnetic field, magnetic particles align in its direction forming aggregates. The mechanical properties of magnetorheological fluids under the presence of an external magnetic field significantly change. In particular, if the particle concentration and field strength is sufficiently large, it becomes necessary to overcome a stress threshold, the so-called yield stress for the onset of flow. Furthermore, under the presence of external fields magnetorheological fluids become highly viscoelastic. The mechanical properties of these materials can be easily and rapidly controlled by the external magnetic field. Due to their controllable mechanical properties, magnetorheological fluids are currently used in several commercial applications concerning vibration control, shock absorbers, precision polishing and even biomedical applications. Currently, for their proper performance, commercial devices require higher yield stresses in their operating modes and the use of highly concentrated magnetorheological fluids. A careful study of the rheological behavior of these systems under compression can be of great utility since it has been shown that the yield stresses under compression are higher than the yield stresses under shear flow mode at the same concentration. An extensive investigation of the magnetorheological properties under compression has been carried out in this dissertation from an experimental point of view, using theoretical developments and performing particle-level simulations. Experimental results showed that both the normal force and the compressive stress increase during the compression test. The dependence with the magnetic field strength was quadratic. The normal force and the yield compressive stress depend linearly on the particle volume fraction in the dilute case and quadratic in the concentrated regime. Due to fact that the magnetic particles employed in the formulation of commercial magnetorheological fluids are typically polydisperse in size, the investigation of the effect of polydispersity in the MR performance is also of interest. Experiments and particle-level simulations were done on particle size distributions having the same average diameters but different polydispersity indexes. The results showed that although the microscopic structure of magnetorheological fluids profoundly changes with the polydispersity, overall, the yield stress does not significantly changes in polydisperse systems.