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Physical Science: Session 1

A Closer Look: Mystery Substance

What is the mystery substance?

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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