Magneto-rheological fluids are dispersions of magnetic micro-particles in a non-magnetic carrier fluid. Their characteristics vary upon the application of a magnetic field. At large shear rates, the Bingham model predicts a linear dependency between the variations of the shear rate and the variations of the shear stress, above a so-called Bingham shear stress threshold. The MRFs behavior below this threshold has received less attention. Its understanding is needed when the fluid is used in human-machine interfaces since small stresses are observed in the transient phases in these kinds of applications. The experimental analysis of the pre-yield regime of the MRF at low shearing rates gives a shearing response ruled by two successive regimes limited by an interfacial phenomenon. In the initial regime, the MRF behaves like a pseudo-elastic material. The shearing pseudo-elasticity was found to be independent of the magnetic field and of the particle volume fraction. Polymers coating of the magnetic particles (revealed by microscopy) is likely to be responsible for this non-magnetic cohesion of the aggregates. The shear-stress limit of this regime is proportional to the square of the magnetic field and to the particle volume fraction. In the next regime, the shear strain is not uniform any more in the fluid. The increase in the measured shear stress varies linearly with the increase in average shear strain. The variation coefficient is proportional to the square of the magnetic field and decreases with the particle volume fraction. Under our experimental conditions this regime was found to be limited by a loss of adhesion between the magnetic aggregates and the shearing plate, when it is non-magnetic, or with the magnetic pole in the other case. The adhesion loss occurs at a threshold proportional to the square of the magnetic field and to the particle volume fraction. This threshold depends on the nature of the shearing plate and may be larger than the Bingham threshold
