Shear-thinning fluid Shear-thickening fluid Shear thickening
Applying a shear force can cause random coils of a polymer to unwind
and become entangled with each other, raising the viscosity. When the
force is removed, the polymer returns to the favored random coil state.
Applying a shear force breaks hydrogen bonds (or other secondary
structures) and allows the polymer strands to flow past each other
more easily. When the force is removed, the hydrogen bonds between
the polymer strands form again.
Newton observed that if a substance is
heated, it will become less viscous, and if
cooled more viscous. When attempting to
start your car on a freezing cold morning,
you may notice that the engine has a difficult
time turning over, because the oil in the car is
thick and sluggish. Once the engine heats up,
the oil becomes less viscous and flows more
Most common fluids, such as water and
oil, are Newtonian. The only factor affecting
their viscosity is temperature, so their viscos-
ity remains constant, no matter how fast they
are forced to flow through a pipe or channel.
But the viscosity of some fluids is affected
by factors other than temperature. These
fluids are termed non-Newtonian fluids. The
viscosity of a non-Newtonian fluid will change
due to agitation or pressure—technically
known as shear stress. A shear stress will not
affect the viscosity of a Newtonian fluid.
Non-Newtonian fluids are polymers. A polymer is composed of long chains of repeated
units known as monomers that are strung
together to yield giant macromolecules. The
most common synthetic polymers are plastics,
rubber, and fabrics, such as polyester and
Spandex. TM Natural polymers include DNA,
proteins, and starch.
Consider ketchup. You may need to tap the
bottom of the bottle or shake it loose to get
it to come out. In doing so, you are applying
a shear stress to the ketchup, causing it to
become less viscous. Fluids that become less
viscous when shaken or stirred are known as
shear-thinning fluids. Other shear-thinning
fluids are shaving cream, toothpaste, and
paint. If you rub shaving cream between your
hands, it will become thin and runny, as its
The next time you brush your teeth, if you
use an electric toothbrush, you will notice that
as the bristles spin, agitating the toothpaste,
the toothpaste twists and flows, becoming
Another shear-thinning fluid, popularized in
movies, is quicksand. If trapped in quicksand,
the more you struggle, the faster you sink.
Rather than helping you to escape, all of your
thrashing about makes the quicksand less
viscous, causing you to sink faster. But don’t
worry—quicksand is seldom deep enough for
you to sink in over your head. And since its
density is about twice that of a person, if you
relax you will float to the top.
Other types of fluids respond in the oppo-
site way to a shear stress. If a shear stress is
applied, they become more viscous. These
fluids are termed shear-thickening. A mix-
ture of corn starch and water is an excellent
example of a shear-thickening fluid. When
squeezed, it appears to solidify. When making
gravy, which uses corn starch, it becomes
thicker when you stir it.
Shear-thickening fluids can be found in your
body. The synovial fluid that coats the joints
in your knees and elbows is shear-thickening.
Usually, this fluid is not very viscous, allowing for free movement of the joints. But if
you bump your knee or elbow on the sharp
corner of a table, the synovial fluid comes to
the rescue, instantly becoming more viscous
and cushioning your knee or elbow from the
D3O® is another type of shear-thickening
fluid. It is composed of a polymer substance
suspended in an oily-type liquid lubricant.
This mixture is classified as a colloid, a mixture in which the dispersed substance is permanently suspended in a dispersing medium.
Other common colloids include fog, whipped
cream, and milk.
When a stress is applied slowly to a shear-thickening fluid, the polymer chains have time
to move out of the way and rearrange themselves, so the viscosity is not affected. But if
a quick stress is applied, the polymer chains
do not have time to rearrange. Instead, they
become entangled, assuming a solid-like consistency, as the viscosity greatly increases.
Imagine many cars trying to quickly leave
through one exit in a parking lot. If everyone
is in a hurry, the cars will become ensnared
in a traffic jam. But if the traffic exits slowly,
there will be time for each car to leave in an
Shear-thinning fluids behave in precisely
the opposite way as shear-thickening fluids.
Whereas a sudden stress will cause a shear-
thickening fluid to harden instantly, a more
Wrap your finger inside
a D3O® material and hit
hard! You won’t hurt your
finger… thanks to D3O. ®