Stick-slip phenomena in the shear flow of highly concentrated suspensions of magnetic microparticles
Résumé
Behavior of yield stress fluids in the vicinity of the yield point has recently attracted much attention in the literature. Most of the macroscopic rheological measurements performed on complex fluids exhibiting yield stress have shown that a stable steady-state flow is impossible for these fluids at small enough values of the shear rate. In most of the cases, the flow curves of these fluids show an initial region where the stress either decreases or remains quasi-constant with shear rate. This is a region of unstable flow, characterized by an evident loss in homogeneity of the flow and deformation fields, which in many cases are accompanied by oscillations of the shear stress at constant shear rate and vice versa. These oscillations of the shear stress (shear rate) at constant shear rate (shear stress) are the most evident manifestation of the existence of stick-slip instabilities. Some well-known yield stress fluids are suspensions of magnetic microparticles, commonly called magnetorheological fluids. In these fluids, interparticle interactions can be induced and tuned over several decades by application of magnetic fields. Unfortunately, flow instabilities like stick-slip, and the effect of the applied field strength in these, have not received proper attention in the literature for these fluids. In view of the lack of studies in this field, the aim of the present work has been to conduct a detailed experimental study on stick-slip phenomena in suspensions of magnetic microparticles. For this aim, we have dealt with the shear flow of highly concentrated suspensions of iron microparticles. We have made measurements in a controlled-rate mode, in rough parallel-plate geometry, and studied the time evolution of the stress response. The influence of different experimental parameters (applied field strength, particle size, gap thickness) have also been studied. We have found saw-tooth-like stress oscillations, typical of stick-slip instabilities, at low enough shear rate values. From the microscopic viewpoint, these stick-slip instabilities happen due principally to periodic failure and healing of the particle structures. At the same time, solid friction between particles should not be excluded and, thus, we also analyze in this work if lubricated contacts between particles (lubricated regime), or direct contacts with solid friction (frictional regime), are present in the systems under study.