Dilatant Materials (Non-Newtonian) and Molecular Bonds
Country: United States
Date: May 2007
Why do dilatant materials "freeze or lock into place" when
subjected to severe stresses? Do bonds temporarily form between the
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
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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Update: June 2012