Più salato il mare: The effect of salt on the shear thickening behavior of non-Brownian suspensions

Dense suspensions of cornstarch grains in water is an object of both popular intrigue and deep scientific inquiry. These non-Brownian suspensions display striking behavior when placed under high stress, namely shear thickening. Although ostensibly simple, the nature of shear thickening in this system has recently been shown to be affected by altering the suspension medium; whether it be by replacing water with ethanol or by adding proteins which modulate the hydrogen bonding between individual grains. Salts are often employed to density match the suspending medium with the starch grains in order to eliminate sedimentation effects from longer timescale experiments. However, the effect of the presence of salts on the flow behavior has not yet been systematically studied. In this talk, I will discuss how we experimentally utilize high speed photography and the free surface geometry of droplet impact in tandem with conventional rheological techniques to ascertain the relationship between salt and macroscopic fluid behavior. We find that the onset of discontinuous shear thickening begins at lower grain concentrations when salts of increasing ionic radius are used. This characteristic is manifested in droplet impact experiments which show droplets who, at the same concentration of grains and same impact stress, portray vastly different impact responses ranging from viscous spreading to partial jamming.