MadSci Network: Physics

Re: What, on a particle level, happens when 'cornstarch slurry' turns solid?

Date: Wed Jan 17 08:39:01 2007
Posted By: Joe Regenstein, Faculty, Food Science
Area of science: Physics
ID: 1168967621.Ph

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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