|MadSci Network: Physics|
One of my colleagues provided the following: The cornstarch slurry is dilatant, which means its viscosity increases with increasing shear rate above a certain critical concentration of cornstarch. But why? At this concentration starch particles touch and interact with each other throughout the container volume and must nudge by each other for flow to occur. At low shear rates the thermal agitation of the particles is faster than the enforced external shear rate timeframe , which ~ as 1/shear rate. The particles easily rearrange themselves and can be stirred. However, as the shear rate increases, and the characteristic time of the process decreases, the "natural time" of the starch slurry becomes greater than 1/shear rate. At this point, all of the individual particles stop any relative motion related to each other, which is required for flow, and "freeze" in place and act as a single large cage structure. This is perceived as solid-like behavior. It is about the relationship between the external forcing timeframe (reciprocal of the rate of shear) and the natural relaxation time of the material at a given temperature that defines this liquid to solid material behavior transition. It helps with the case of starch that the ungelatinized particles are themselves more or less rigid bodies. You might Google the "Deborah Number" for more general info on this topic.
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