Dense suspensions: Frictional effects in
shear thickening and related phenomena
Jeffrey F. Morris
Levich Institute and Dept. of Chemical Engineering
CUNY City College of New York
Suspensions of solid particles in liquids have a wide range of applications, from coatings to cement, and appear in nature as mud flows and sediment transport. Their properties vary widely, from low-viscosity liquids to wet granular materials or soft solids, depending on the solids loading. When the particles are very concentrated, these mixtures are often called “dense suspensions”. Dense suspensions often exhibit shear thickening, an increase in apparent viscosity as the shear rate is increased. When conditions are right, abrupt order of magnitude increases in viscosity over a narrow range in shear rate occur, and this is termed the discontinuous shear thickening (DST); this is seen in dispersions of particles from far below a micron in diameter to tens of microns, with corn starch in water the well-known example.
Using numerical simulations, we will establish a scenario in which the DST phenomenon, and the related large normal stress response, is explained by the transition from lubricated to frictional interactions between particles as the driving shear stress increases. This complex rheology results from the interplay of viscous lubrication, repulsive surface forces, and contact with frictional interactions between particle surfaces. The simulation method will be outlined, with results compared to efforts by other groups, including experimental studies and a theory incorporating the same basic elements as these simulations.