Keshvad Shahrivar

Two-step yielding in magnetorheology

We use particle level dynamic simulations, ultrasonic characterization, and rheomicroscopy to investigate the yielding behavior of magnetorheological (MR) fluids under oscillatory shear in both dilute and concentrated regimes. Dilute suspensions exhibit a single peak in the elastic stress that is associated to the breaking of the field-induced structures at the flow point (G′ = G″). On the other hand, more concentrated suspensions demonstrate a two-step yielding that is associated to the existence of short-ranged attractions between the particles, possibly coming from remnant magnetization or van der Waals forces. This two-step yielding is demonstrated by introducing additives in the formulation of the MR fluids and performing particle level simulations that include R-shifted Lennard-Jones potentials of interaction.

Thermoresponsive polymer-based magneto-rheological (MR) composites as a bridge between MR fluids and MR elastomers

A new class of magnetorheological (MR) material is reported that bridges the gap between conventional MR fluids and MR elastomers. The key point is the use of a thermoresponsive polymer-based suspending medium in the formulation of the MR material whose rheological properties can be externally controlled through changes in the temperature. Under appropriate conditions, it is even possible to induce a thermally driven “liquid-to-solid” transition in the carrier medium. As a result, when the suspending medium is in the “liquid” phase, the MR composite behaves as a conventional MR fluid exhibiting a large MR effect (approx. 1000%). In contrast, when the suspending medium is in the “solid” phase and exhibits an apparent yield stress, the whole material behaves as a MR elastomer where the iron microparticles are entrapped in a polymer network. Two different examples are reported in this communication; on the one hand a triblock copolymer solution is used as a suspending medium that evolves towards a lyotropic liquid crystalline cubic phase upon heating. On the other hand, a concentrated microgel dispersion is used that reaches maximum packing as the temperature decreases forming a repulsive colloidal glass. The rheological properties of the materials are evaluated under dynamic oscillatory and steady simple shear to ascertain the MR effect.

Thermogelling magnetorheological fluids

A novel approach is proposed for the formulation of kinetically stable magnetorheological (MR) fluids exhibiting an MR effect. Thermoresponsive carrier fluids are used which develop a sol–gel transition on increasing the temperature. Turbidity measurements, multiwave rheology and steady shear flow tests are carried out on model conventional MR fluids prepared by dispersion of carbonyl iron microparticles in triblock copolymer solutions of type PEOx–PPOy–PEOx with x = 100 and y = 65. Experiments demonstrate that the MR fluids remain stable against sedimentation in the gel phase and exhibit a very large (relative) MR effect (up to 1000%) in the sol phase.

Creep and recovery of magnetorheological fluids: Experiments and simulations

A direct comparative study on the creep-recovery behavior of conventional magnetorheological (MR) fluids is carried out using magnetorheometry and particle-level simulations. Two particle concentrations are investigated ( ϕ=0.05 and 0.30) at two different magnetic field strengths (53 and 173 kA·m−1) in order to match the yield stresses developed in both systems for easier comparison. Simulations are mostly started with random initial structures with some additional tests of using preassembled single chains in the low concentration case. Experimental and simulation data are in good qualitative agreement. The results demonstrate three regions in the creep curves: (i) In the initial viscoelastic region, the chainlike (at ϕ=0.05) or percolated three-dimensional network (at ϕ=0.30) structures fill up the gap and the average cluster size remains constant; (ii) Above a critical strain of 0.1 (10%), in the retardation region, these structures begin to break and rearrange under shear. At large enough imposed stres...

Ferrofluid Lubrication of Compliant Polymeric Contacts: Effect of Non-homogeneous Magnetic Fields

This paper demonstrates a new route to control friction in the isoviscous elastic lubrication regime between compliant point contacts. For this aim, the superposition of non-homogeneous magnetic fields in ferrofluid lubricated contacts is proposed. Under appropriate conditions, a friction reduction is observed by simply displacing the magnetic field distribution in the flow direction towards the inlet of the contact. This friction reduction is associated with a lower Couette friction contribution as a result of the fact that surfaces become more separated under field. Experiments and simulations are in good qualitative agreement.

Magnetorheology of hybrid colloids obtained by spin-coating and classical rheometry

Hybrid colloids composed of micron-sized ferromagnetic (carbonyl iron) and diamagnetic (silica) particles suspended in cyclohexanone, behave as, non-Newtonian, magnetorheological fluids. We measure and compare the magnetic field-dependent viscosity of hybrid diluted colloids using spin-coating and conventional magnetorheometry. We extend a previously developed model for simple colloids to this kind of hybrid colloids. As in the previous model, the viscosity of the colloidal suspension under applied fields can be derived from the surface coverage of the dry spin-coated deposits for each type of particles, and from the viscosity of the colloid at zero field. Also, our results allow us to obtain the evaporation rate of the solvent as a function of the rotation speed. Finally, we also measure the viscosity of the same suspension with a torsional parallel plate magnetorheometer under uniaxial DC magnetic fields aligned in the velocity gradient direction of a steady shearing flow. The experimental results under different conditions and the effect of each component on the magnetorheological properties of the resulting colloid are discussed. Standard spin-coating technique can be used both to characterize smart materials—complex fluids as well as to fabricate films with arbitrary solvents by tuning their viscosity by means of external fields.