Dynamic Behavior of Discontinuous Shear Thickening Fluid

Discontinuous shear thickening fluids are non-Newtonian fluids that behave as a liquid in general but show solid-like properties under shear or impact. It also shows a number of interesting dynamic phenomena such as the ability of a person to run on the surface of cornstarch and water, the oscillatory behavior of a ball sinking in a pool of cornstarch and water, formation of stable holes and fingers under vibration, etc. In this work, we experimentally investigate the behavior of these shear thickening fluids under impact using a model system of cornstarch suspensions in water. We find that these suspensions can support very large stresses, in the order of MPa, under impact - with a time delay once the impactor hits the surface. We show that neither the delay nor the large stresses can be explained by standard rheological models, or other proposed impact-based models - with added mass, and inertial effects. These large stresses are observed when a dynamically jammed region forms below the impactor and propagates to the boundary of the container. During this, the dynamically jammed region behaves like a solid, and can support these large stresses. We present an average constitutive relationship for this impact rheology that relates the force on the impactor to it's displacement in terms of the delay time, and an effective modulus of the solid-like region as a function of impact velocity, sample height, and sample weight fraction.;We also investigated the transient relaxation of these DST suspensions in shear rheology experiments. We performed two types of experiments, starting from a steady shear in a parallel plate rheometer followed by either stopping the top plate rotation and measuring the torque relaxation, or removing the torque on the top plate and measuring the transient rotation of the tool. We found that at lower effective weight fractions, the suspensions exhibited a relaxation behavior consistent with a generalized Newtonian fluid. However, at higher weight fraction, the relaxation behavior differs both qualitatively and quantitatively from the Newtonian model. We find that at an effective weight fraction &phis; < &phis;c , the relaxation time starts deviating from the generalized Newtonian prediction and starts to diverge from the prediction. At weight fractions close to the liquid-solid transition, the discrepancy is as large as ~104 s. This difference in the model prediction versus the data shows that the relaxation behavior is not shown by the dissipative terms in the constitutive model. It is also at this high weight fraction range, that we measured relaxation times in the order of ~1s. This time scale is easily resolvable by eye, and may be important in explaining some of the dynamic phenomenon associated with cornstarch water suspensions. We also showed that an effective weight fraction derived from critical shear rate at the onset of shear thickening can more precisely characterize the material properties near the critical liquid-solid transition point. This effective weight fraction scale can be beneficial in comparing experiments done at different lab conditions.;We also present state transitions observed in shear thickening fluids, showing that the normal stress goes from negative to positive at a weight fraction different from the continuous- to discontinuous- shear thickening transition point, which also is distinct from the transition points of relaxation time divergence and liquid-solid transition. Further, we show a method to measure dilation seen on the surface during shear experiments, and show how the weight fraction where dilation becomes visible compares to all of these state transitions observed.