Lumpy Liquids and Squishy Solids
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Have you ever turned a liquid into a solid just by tapping on it? In this
experiment you make just such a liquid.
Materials:
Place a sheet of newspaper flat on a table. Put the mixing bowl in the
middle of the newspaper. Add ¼ cup of dry cornstarch to the bowl.
Add about 1/8 cup (2 tablespoons, or 30 cm3) of water to the corn
starch and stir slowly. Add water slowly to the mixture, with stirring,
until all of the power is wet.
Continue to add water until the cornstarch acts like a liquid when you
stir it slowly. When you tap on the liquid with your finger, it shouldn't
splash, but rather will become hard. If your mixture is too liquid, add
more cornstarch. Your goal is to create a mixture that feels like a stiff
liquid when you stir it slowly, but feels like a solid when you tap on it
with your finger or a spoon.
Scoop the cornstarch mixture into the palm of your hand, then slowly
work it into a ball. As long as you keep pressure on it by rubbing it
between your hands, it stays solid. Stop rubbing, and it “melts” into a
puddle in your palm. Can you think of other tests you can do with it?
Why does the cornstarch mixture behave like this?
Think of a busy sidewalk. The easiest way to get through a crowd of
people is to move slowly and find a path between people. If you just
took a running start and headed straight for the crowd of people, you
would quickly slam into someone and you wouldn't get very far. This
is similar to what happens in the cornstarch mixture. The solid
cornstarch acts like a crowd of people. Pressing your finger slowly
into the mixture allows the cornstarch to move out of the way, but
tapping the mixture quickly doesn't allow the solid cornstarch
particles to slide past each other and out of the way of your finger.
We use the term “viscosity” to describe the resistance of a liquid to
flow. Water, which has a low viscosity, flows easily. Honey, at room
temperature, has a higher viscosity and flows more slowly than
water. But if you warm honey up, its viscosity drops, and it flows
more easily. Most fluids behave like water and honey, in that their
viscosity depends only on temperature. We call such fluids
“Newtonian,” since their behavior was first described by Isaac
Newton (when he wasn’t discovering the laws of gravity or
developing the calculus). The cornstarch mixture you made is called
“non-Newtonian” since its viscosity also depends on the force
applied to the liquid or how fast an object is moving through the
liquid.
Other examples of non-Newtonian fluids include ketchup, silly putty,
and quicksand. Quicksand is like the cornstarch mixture: if you
struggle to escape quicksand, you apply pressure to it and it
becomes hard, making it more difficult to escape. The
recommended way to escape quicksand is to slowly move toward
solid ground; you might also lie down on it, thus distributing your
weight over a wider area and reducing the pressure. Ketchup is the
opposite: its viscosity decreases under pressure. That’s why
shaking a bottle of ketchup makes it easier to pour.
Disposal: First dilute the cornstarch mixture with plenty of water
before pouring it down the drain. Why? What do think would happen
to the semi-solid, semi-liquid form that you prepared if pressure
were applied to it by other water in the drain? Yes – a plugged drain.
Special Thanks
Thanks to http://scifun.chem.wisc.edu for providing the experiment
instructions.
experiment you make just such a liquid.
Materials:
- A bowl for mixing
- newspaper
Place a sheet of newspaper flat on a table. Put the mixing bowl in the
middle of the newspaper. Add ¼ cup of dry cornstarch to the bowl.
Add about 1/8 cup (2 tablespoons, or 30 cm3) of water to the corn
starch and stir slowly. Add water slowly to the mixture, with stirring,
until all of the power is wet.
Continue to add water until the cornstarch acts like a liquid when you
stir it slowly. When you tap on the liquid with your finger, it shouldn't
splash, but rather will become hard. If your mixture is too liquid, add
more cornstarch. Your goal is to create a mixture that feels like a stiff
liquid when you stir it slowly, but feels like a solid when you tap on it
with your finger or a spoon.
Scoop the cornstarch mixture into the palm of your hand, then slowly
work it into a ball. As long as you keep pressure on it by rubbing it
between your hands, it stays solid. Stop rubbing, and it “melts” into a
puddle in your palm. Can you think of other tests you can do with it?
Why does the cornstarch mixture behave like this?
Think of a busy sidewalk. The easiest way to get through a crowd of
people is to move slowly and find a path between people. If you just
took a running start and headed straight for the crowd of people, you
would quickly slam into someone and you wouldn't get very far. This
is similar to what happens in the cornstarch mixture. The solid
cornstarch acts like a crowd of people. Pressing your finger slowly
into the mixture allows the cornstarch to move out of the way, but
tapping the mixture quickly doesn't allow the solid cornstarch
particles to slide past each other and out of the way of your finger.
We use the term “viscosity” to describe the resistance of a liquid to
flow. Water, which has a low viscosity, flows easily. Honey, at room
temperature, has a higher viscosity and flows more slowly than
water. But if you warm honey up, its viscosity drops, and it flows
more easily. Most fluids behave like water and honey, in that their
viscosity depends only on temperature. We call such fluids
“Newtonian,” since their behavior was first described by Isaac
Newton (when he wasn’t discovering the laws of gravity or
developing the calculus). The cornstarch mixture you made is called
“non-Newtonian” since its viscosity also depends on the force
applied to the liquid or how fast an object is moving through the
liquid.
Other examples of non-Newtonian fluids include ketchup, silly putty,
and quicksand. Quicksand is like the cornstarch mixture: if you
struggle to escape quicksand, you apply pressure to it and it
becomes hard, making it more difficult to escape. The
recommended way to escape quicksand is to slowly move toward
solid ground; you might also lie down on it, thus distributing your
weight over a wider area and reducing the pressure. Ketchup is the
opposite: its viscosity decreases under pressure. That’s why
shaking a bottle of ketchup makes it easier to pour.
Disposal: First dilute the cornstarch mixture with plenty of water
before pouring it down the drain. Why? What do think would happen
to the semi-solid, semi-liquid form that you prepared if pressure
were applied to it by other water in the drain? Yes – a plugged drain.
Special Thanks
Thanks to http://scifun.chem.wisc.edu for providing the experiment
instructions.