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

We explore the rheology of very concentrated (0.55<F<0.67) suspensions of carbonyl iron (CI)
particles coated by a small polymer. A strong discontinuous shear thickening (DST) is observed
in a large range of volume fraction presenting some specific behaviours in comparison to 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 behaviour is observed in the presence of a magnetic field with a strong
increase of the viscosity before the DST transition. We interpret this behaviour 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 behaviour.