Discontinuous Shear Thickening of Suspensions of Magnetic Particles in Relation to the Polymer Coating on Their Surfaces

The phenomenon of discontinuous shear thickening (DST) is observed in suspensions of solid particles with a very high-volume fraction. It is characterized by an abrupt decrease in the shear rate for critical stress during a ramp of stress. This behavior can be reproduced in numerical
simulations by introducing a local friction between two particles above a given local force. We
present experimental results showing this DST behavior obtained with suspensions of magnetic (iron) and nonmagnetic (calcium carbonate) particles and different amounts of a superplasticizer molecule used in the cement industry. For both types of particles, the same behavior was observed
with first an increase in critical stress with the amount of plasticizer followed by a decrease at higher
concentrations but with a larger viscosity before critical stress was reached. At a low concentration
of plasticizer, the low critical stress is interpreted by the local sliding of plasticizer molecules on the
surface of particles. At higher concentrations, when total coverage is achieved, the critical stress is
higher since it has to remove the molecules out of the surface. At still higher concentrations, the
increase in viscosity is explained by the formation of multilayers of molecules on the surface of the
particles. This interpretation is supported by the measurement of the adsorption isotherm of the
plasticizer on the surface of the particles.