Dynamics of magnetorheological fluids at the microscale

Abstract

In this dissertation, we investigated a new class of MRFs that bridges the gap between conventional MRFs and MR elastomers. In a novel approach, a thermoresponsive polymer-based suspending medium, whose rheological properties can be externally controlled through changes in the temperature, was used in the formulation of the MRFs. We used, particularly, Poly (ethylene oxide)–poly (propylene oxide)–poly (ethylene oxide) triblock copolymers and Poly(N-isopropylacrylamide) microgels. Thus, we found a feasible way to prevent particle sedimentation in MRFs but at the same time retaining a very large MR effect in the excited state. The study of the creep flow behavior of MRFs is of valuable help in understanding the yielding behavior of these materials. A direct comparative study on the creep-recovery behavior of conventional MR fluids was carried out using magnetorheometry and particle-level simulations. We show that the recovery behavior strongly depends on the stress level. For low stress levels, below the bifurcation value, the MRF is capable to recover part of the strain. For stresses larger than the bifurcation value, the recovery is negligible as a result of irreversible structural rearrangements. From a practical point of view, it is interesting to study the thin-film rheological and tribological properties of FFs. Recently, it has been shown that by using FFs in mechanical contacts it is possible to actively control the frictional behavior. In this dissertation, we explored a new route to control friction in the isoviscous-elastic lubrication regime between compliant point contacts. By superposition of nonhomogeneous magnetic fields in FFs lubricated contacts, a friction reduction was achieved. Also, we compared the tribological performance of FFs and MRFs using the same tribological conditions and tribopairs. In the case of FFs lubrication the sliding wear occurs mainly by two-body abrasion and in the case of MRFs lubrication the sliding wear occurs by two-body and three-body abrasion. Finally, the study of the growing rate of the field-driven structure formation is also important, in particular, for the prediction of the response time of MRFs since their practical applications depend on the rate of change in their properties. The irreversible two-dimensional aggregation kinetics of dilute non-Brownian magnetic suspensions was investigated in rectangular microchannels using video-microscopy, image analysis and particle-level dynamic simulations. Especial emphasis was given to carbonyl iron suspensions that are of interest in the formulation of MRFs; carrier fluid viscosity, particle/wall interactions, and confinement effect was investigated. On the one hand, the carrier fluid viscosity determines the time scale for aggregation. On the other hand, particle/wall interactions strongly determine the aggregation rate and therefore the kinetic exponent. It was found that aggregation kinetics follow a deterministic aggregation process. Furthermore, experimental and simulation aggregation curves can be collapsed in a master curve when using the appropriate scaling time. The effect of channel width is found to be crucial in the dynamic exponent and in the saturation of cluster formation at long times. On the contrary, it has no effect in the onset of the tipto- tip aggregation process.