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 (phi = 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 phi = 0.05) or percolated three-dimensional network (at phi = 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 stress values, they transform into thin sheetlike or thick lamellar structures, depending on the particle concentration; (iii) Finally in the case of larger strain values either the viscosity diverges (at low stress values) or reaches a constant low value (at high stress values), showing a clear bifurcation behavior. For stresses below the bifurcation point, the MR fluid is capable to recover the strain by a certain fraction. However, no recovery is observed for large stress values. (C) 2014 The Society of Rheology.