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Name: Jonathan
Status: Student
Grade:  9-12
Location: CA
Country: United States
Date: May 2007


Question:
Why do dilatant materials "freeze or lock into place" when subjected to severe stresses? Do bonds temporarily form between the molecules?



Replies:
Typically, shear thickening occurs because the stress causes some structure or order to occur. Imagine you are trying to work your way through a crowded room (or to the front row of music concert). If you just ram your way through, people will resist and push you back. If you go slowly, you can find the holes and spaces between people and work your way through them. It works in a similar fashion with shear thickening fluids.

I suppose when you push certain particles together, they might form weak bonds such as dipole interactions, but I am not aware of any fluids that form and break covalent bonds. Some fluids -- silly putty comes to mind -- have cross-linkers that form bonds that can be broken and reformed, but that material is a shear thinning fluid (a combo elastic and shear-thinning fluid, actually).

Burr Zimmerman


Dilitant materials (the most commonly well-known one is a mixture of corn starch and water which makes it possible to walk on "water") are able to "lock in place" because the material responds to the intensity or frequency of the stress (or force) applied to it.

Imagine a loose ball of twine. If you were to pull on a single strand of twine in this ball, and you were to do this gently, it is possible to unravel the ball. However, if you were to pull on the thread really fast, you are more likely to cause a knot to form and the ball tightens up. This is essentially what happens at the molecular level for dilitant materials. High frequency or high intensity forces tends to make the polymeric, long-stranded polymers to tangle up tightly and cause the whole system to seize up. But a slow application of force, or a weak force, allows the polymer strands to rotate and flow around the force so that the material remains relatively fluid.

Greg (Roberto Gregorius)



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