Dynamics of magnetorheological fluids at the microscale

Resumen

Materials whose properties change in the presence of an external stimulus are known as smart materials. Magnetic fluids are smart colloidal suspensions of ferromagnetic/ferrimagnetic particles whose rheological properties can be tuned by a magnetic field. Generally speaking, magnetic fluids can be divided in two categories: ferrofluids (FFs) and magnetorheological fluids (MRFs). MRFs are suspensions of magnetizable micron-sized particles suspended in a non-magnetic medium and FFs are dispersions of nano-sized particles in a non-magnetic carrier fluid. Under the application of a magnetic field, in the case of MRFs, magnetized particles form aggregates along the field direction and the apparent viscosity changes in several orders of magnitude [1]. If the magnetic field is large enough, a minimum force is needed to make the suspension flow (i.e. yield stress). Under external magnetic fields, MRFs acquire viscoelastic properties and exhibit a strongly shear-thinning behavior. Being field responsive suspensions, MRFs and FFs have received considerable attention in the field of mechanical engineering for actuation and motion control. Some examples are shock absorbers, dampers, clutches and rotary brakes [2]. However, interest from other research areas such as thermal energy transfer, precision polishing, chemical sensing and biomedical applications shows the potential of these materials to be employed in many other different disciplines. 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 [3]. In this dissertation, we explored a new route to control friction in the isoviscous-elastic lubrication regime between compliant point contacts. By superposition of non-homogeneous 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 [4]. 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 tip-to-tip aggregation process. Conclusions In this dissertation, thermoresponsive materials have been used in the formulation of a new class of MRFs. The creep-recovery behavior of MRFs has been investigated experimentally and via Brownian Molecular Dynamic Simulations. Also, the tribological properties of FFs and MRFs have been studied. Furthermore, the aggregation kinetics of carbonyl iron particles was investigated in PDMS microchannels. Main conclusions are summarized as follows: Thermoresponsive MRFs 1. The use of thermoresponsive materials in the formulation of MRFs constitutes a feasible way to prevent particle sedimentation, but at the same time, retaining a very large MR effect in the excited state. 2. When the carrier operates in the "liquid" phase, the MR composite behaves as a conventional MR fluid. On the contrary, in the "solid" phase, the MR composite behaves as a conventional MR elastomer. 3. Below (above) the sol/gel transition temperature of a concentrated microgel suspension (of a triblock copolymer solution), the MR fluid exhibits a large MR effect as a result of a very low viscosity carrier fluid. 4. Above (below) the sol/gel transition of a concentrated microgel suspension (of a triblock copolymer solution), the dynamics of the iron particles is arrested and sedimentation is inhibited. Creep-recovery of MRFs 1. Independently of the particle loading, three regimes are observed in the creep curves: i) Initial response regime where the systems behave in the viscoelastic region and the average cluster size remains constant. ii) Retardation regime where the system behaviour results from the balance between the applied stress and the stress contributions from the particle interactions and viscous flow. iii) Long-time steady state where structural parameters reach plateau values. In the case of large enough stress values the system enters in a constant strain rate regime and viscosity bifurcation occurs. 2. The recovery behavior strongly depends on the stress level. For low stress levels, below the bifurcation value, the MR fluid is capable to recover part of the strain. For stresses above the bifurcation value, the recovery is negligible as a result of irreversible structural rearrangements. 3. Long-time creep simulation data are consistent with steady shear flow. Experiments and simulations compare well for dilute system. 4. For concentrated system, the agreement between experiments and simulations is not so good because this system is more sensitive to interparticle (remnant magnetization and colloidal) forces. Tribological properties of FFs and MRFs 1. In the absence of magnetic fields, the tribological behaviour of FFs can be described in terms of a Stribeck curve that closely matches the one obtained for Newtonian fluids. 2. In the presence of magnetic fields, and for sufficiently small loads, friction decreases with respect to the no-field friction coefficient. 3. Friction coefficient can be further decreased when a non-uniform field distribution is displaced towards the inlet of the contact. 4. For the FFs lubrication, the sliding wear occurs mainly by two-body abrasion but in the case of MRFs lubrication the sliding wear occurs by two-body and three-body abrasion. Aggregation kinetics 1. The carrier fluid viscosity determines the time scale for the aggregation process. 2. When particles interact strongly with the wall, aggregation is hindered and the mean cluster size remains essentially constant at a low value. 3. When friction is reduced the mean cluster size increases following a deterministic aggregation process. 4. Experimental and simulation curves can be collapsed in a master curve, in the power-law region, when using the time scale obtained from particle-level (non-Brownian) dynamic simulations in agreement with a ballistic aggregation mechanism. 5. The plateau region in the mean cluster size distribution at long times is as a result of the arrest of the aggregation kinetics because of the presence of the confining walls. References [1] J. de Vicente, D. J. Klingenberg, and R. Hidalgo-Alvarez, “Magnetorheological fluids: a review,” Soft Matter, vol. 7, no. 8, p. 3701, 2011. [2] N. Wereley, Magnetorheology: advances and applications, vol. 6. Royal Society of Chemistry, 2013. [3] E. Andablo-Reyes, J. De Vicente, R. Hidalgo-Álvarez, C. Myant, T. Reddyhoff, and H. A. Spikes, “Soft elasto-hydrodynamic lubrication,” Tribol. Lett., vol. 39, no. 1, 2010. [4] M. Mohebi, N. Jamasbi, and J. Liu, “Simulation of the formation of nonequilibrium structures in magnetorheological fluids subject to an external magnetic field,” Phys. Rev. E, vol. 54, no. 5, pp. 5407–5413, 1996.