OObleck - a Non-Newtonian fluid
From Dr Seuss's book "Bartholomew and The Oobleck" we know that
Oobleck is green nasty substance that was called down from the sky by a king bored with ordinary rain and snow. Oobleck proved so sticky
that it gummed up the whole kingdom.
The word has since been used in science to describe a substance that exhibit properties of liquids and solids at the same time.
This oobleck is created from cornstarch (1 part water to 1.5–2 parts
cornstarch, depends on what kind you want to have. More starch,
Though initially it acts like a liquid or a jelly, squeezing it in your hand will make it appear to be a solid for a short time. The slime-like substance also behaves in an interesting manner when thrown in the air, molded, heated, or vibrated. Substances like this that become more viscous when agitated or compressed are a subset of non-Newtonian fluids called dilatants. What defines the dilatant is that as a force is applied to the fluid the viscosity (a measure of how difficult it is for the fluid to flow in a way) increases.
Fluids of this sort are being researched for bullet resistant
body armor, useful for their ability to absorb the energy of a high velocity projectile impact but remain soft and flexible while worn. 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 force
OObleck - a Non-newtonian fluid (procedure)
Measure 1 1/2 cups of cornstarch and put in a pie pan
or container, If you want a color of Oobleck add the
coloring to the water first. Then gradually add
approximately 1/2 cup of water to the cornstarch. Stir
well (this will take some time). Add small amounts of
more water or cornstarch until you get a mixture which
'tears' when you quickly scrape your finger through it
AND THEN 'melts' back together again.
A non-Newtonian fluid is a fluid in which the
viscosity changes with the applied strain rate.
Oobleck is often referred to as a 'non-Newtonian' substance because it does not
behave as Newton's Third Law of Motion states; for every action, there is an
equal and opposite reaction. Applying this principle, you would expect Oobleck
to 'splash' when you 'smack' it with your hand. (Smacking is the action,
splashing is the reaction.) However, when you try this out Oobleck does not splash, in fact, it becomes a solid substance for a few moments.
Why? Scientists explain this as follows. Uncooked corn starch particles are structured in both crystalline and noncrystalline arrangements. When slowly mixed with water, the non crystalline structures of corn starch absorb most of the water. When you smack or stir it rapidly, you increase the temperature and pressure on the mixture which causes more noncrystalline structures to form. These new noncrystalline structures absorb more water and the mixture becomes thicker: hence the appearance of a solid. When you discontinue the pressure, the number of noncrystalline structures decrease and water is released, creating the 'soupy' mixture. In one possible model, the starch molecules are compared to sand and water in a plastic squeeze bottle. The grains of sand are closely packed with a little water in between. The water’s surface tension doesn’t allow all the space between the sand grains to be filled with sand. Squeezing the bottle gently forces the sand grains to move against each other. This increases the spaces and allows more water to fill the spaces. The more gently you squeeze the more time there is for the water to fill the spaces and provide lubrication. But if you squeeze the bottle quickly, there isn’t enough water between the sand grains and friction between the sand grains resists the flow.
Although sand grains are much larger than molecules of starch, starch molecules are quite large and the mix of cornstarch and water may react very much like a mixture of sand and water. This is one explanation for why Oobleck flows like a fluid, but reacts as a solid when suddenly compressed.
Other scientists base their Oobleck models on chemistry. Cornstarch is made of long chain molecules called polymers. When water is added to cornstarch and the mixture is compressed, the molecules become "tangled" and are unable to slide easily against one another.
A third model suggests that starch molecules acquire an electric charge as they rub together. The faster they are rubbed, the more electrical attraction is created among the molecules. This causes the increase in viscosity.