Discontinuous shear thickening (DST) transition with spherical iron particles coated by adsorbed brush polymer

Abstract

We explore the rheology of very concentrated (0.55 < Φ < 0.67) suspensions of carbonyl iron particles coated by a small polymer. A strong discontinuous shear thickening (DST) is observed in a large range of volume fraction presenting some specific behaviors in comparison with other systems. In particular, the DST transition can appear suddenly without being preceded by shear thickening. The presence of a frictional network of particles is confirmed by a simultaneous measurement of the electric resistance of the suspension and of the rheological curve. Using the Wyart–Cates (W–C) model, we show that with increasing the volume fraction, the fraction of frictional contacts increases more quickly with the stress, contrary to the prediction of numerical simulations. The same behavior is observed in the presence of a magnetic field with a strong increase in the viscosity before the DST transition. We interpret this behavior by the interpenetration of the polymer layer under the effect of the shear stress—and of the magnetic stress—followed by the expulsion of the polymer out of the surfaces between two particles in contact. We point out that above the DST transition, we do not observe a jamming in the range of volume fraction, whereas it is predicted by the W–C model. The frictional contacts are created by a shear stress and not by a static stress, so in the absence of shear flow, the polymer can adsorb again on the surface and lubricate the frictional contacts. We thus predict an asymptotic non-zero shear rate reproducing the experimental behavior.