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A Closer Look: Mystery Substance
 What
is the mystery substance?
The mystery substance that Chris Bash’s class investigates in the
video is a variation on what is also sometimes called “oobleck,” after
the Dr. Seuss’ book Bartholomew and the Oobleck. It is made up of
tiny particles of liquid starch and glue suspended in water. Chemists call
this type of mixture a colloid. As Chris’s students discover, this
colloid behaves strangely. They describe it as sticky, egg-like, slimy,
stringy, droopy, and marshmallow-like. And, as our hosts Sallie and Robin
point out, it has properties of both a solid and a liquid. It “holds
together” if pressed into a ball, but when left alone, it will take
on the shape of its container. Another interesting feature of the mystery
substance is that if you try to stir it slowly, it flows easily, like a
liquid. However, if you try to stir it quickly or strike it sharply, it
resists strongly, like a solid.
Why does the mystery substance behave as
it does?
When you stir a liquid, you are applying what a physicist
would call a sideways shearing force to the liquid. In response, the
liquid shears,
or moves out of the way. The behavior of the mystery substance relates
to its viscosity, or resistance to flow. Water's viscosity doesn't change
when you apply a shearing force — but the viscosity of the mystery
substance does. Back in the 1700s, Isaac Newton identified the properties
of an “ideal” liquid as having a having a consistent viscosity,
or resistance to flow, at any given temperature. Water and other liquids
that have the properties that Newton identified are called Newtonian
fluids. The mystery substance doesn't act like Newton's ideal fluid,
and is therefore
called a non-Newtonian fluid.
Are there other non-Newtonian fluids?
There are many
non-Newtonian fluids. They don't all behave like the mystery substance,
and each one is unique in its own way. Ketchup,
for example,
is a non-Newtonian fluid. Quicksand is a non-Newtonian fluid that
acts more like the mystery substance — it gets more viscous when
you apply a shearing force. If you ever find yourself sinking in a pool
of quicksand
(or a vat of cornstarch), try swimming toward the shore very slowly.
The slower you move, the less the quicksand or cornstarch will resist
your
movement.
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