Oobleck: Is It a Solid or a Liquid?
[Shopping List: Cornstarch; water; bowls or pans; spoons; straws; hammer; scissors; towels. Optional: ketchup (in bottle); slime or Silly Putty (from another activity station); toothpaste; corn syrup; latex paint; paper plates; electric toothbrush; vegetable oil. ]
- In a large bowl or pan, mix cornstarch with a small amount of water, adding the water slowly as you mix with a spoon. It will be very difficult to stir at first, but resist the temptation to add too much water.
- The mixture will eventually become easier to stir as you reach the final ratio of about 1 part water to 3 parts cornstarch. Note that the "oobleck" fluid will only stir easily if you move the spoon very slowly and gently, with almost no force at all. You can even stir it with a straw. If you move the spoon quickly or apply a lot of force, however, it will instantly thicken and resist stirring, behaving as a solid. As soon as you relax the spoon the oobleck will return to more liquid-like behavior.
- Smack the surface of the oobleck very quickly with the flat bottom of the spoon, your fingers or fist, or a hammer. Note that the harder and faster you strike the surface, the more it will resist penetration, but if you simply lay the spoon on the surface it will easily sink to the bottom of the pan.
- Slowly and gently push your fingers into the oobleck, allowing them to gently and easily sink to the bottom of the pan. Now quickly pull your fingers out or move them sideways. The entire pan and all of the oobleck should lift or move with your hand as it becomes solid. If you squeeze your fingers together quickly you can tear the oobleck and open a hole to see the bottom of the pan.
- Put a glob of oobleck in your hands and quickly pat and roll it into a ball. As long as you are moving the ball it will remain solid, but as soon as you stop, it will become a liquid and ooze through your fingers and drip back into the pan.
- Pour some oobleck from a spoon or a small cup back into the pan and observe how it drips and flows. You can even break or cut the oobleck with scissors as it pours.
What's Happening: A fluid is not a phase or state of matter (such as a solid, liquid or gas) but rather any substance that will deform or flow when a shear stress (force) is applied. If you pour some water onto a plate to form a small puddle then lift one edge slightly, the water will slide or flow along the surface of plate due to the shear (parallel to the surface of the plate) stress caused by gravity. The term fluid and liquid are often used a synonyms, but a fluid can actually be liquid, gas, a mixture of either or both, a mixture of a solid in a liquid (a suspension or colloid), or even a solid. If it flows as a shear stress is applied- perhaps very slowly- it can be considered to be a fluid. Many materials thought to be solid- such as glass or tar pitch- will actually flow very slowly, often taking years for any noticeable change to occur.
Most common fluids flow faster as higher shear stress is applied- i.e. the harder you push it the faster it moves. Resistance to movement or flow is called viscosity- a fluid with low viscosity will flow easily even under very low shear stress (an example is water), while a highly viscous fluid flows very slowly (such as molasses). If the flow rate or viscosity is simply proportional to the stress and stops flowing when there is no stress, the substance is called a Newtonian fluid (after Isaac Newton, who studied them). Oobleck, however, is an example of a (very) non-Newtonian fluid. It's flow rate decreases dramatically (i.e. the viscosity increases dramatically) as the sheer stress increases. The harder you hit or pull on the oobleck, the more viscous or solid-like it becomes. You can even run across the surface of a large pool filled with oobleck (many videos on the internet), but if you stop you will sink. This specific type of non-Newtonian fluid is called a stress thickening or dilatant fluid.
More advanced dilatant materials are now being used in protective gear for football and other impact sports. The pads flex and move easily under normal motion, but when subjected to a sudden blow (a hit or tackle) they instantly stiffen and absorb the force of the impact, protecting the athlete. Bullet-proof vests are also being developed with these materials.
To understand what is happening inside the oobleck, picture the solid cornstarch molecules suspended or floating around in the water. As long as little or no stress is applied, the cornstarch molecules are free to move easily (sort of lubricated by the water), sliding over and around each other. In part because there is so much more cornstarch than water in the mixture however, when a large or sudden force is applied the solid cornstarch molecules instantly clump or stick together (flocculate) and the entire matrix acts like a solid. If the mixture is made with a much higher proportion of water, this behavior is not observed.
Variations and Related Activities: There are many other different types of non-Newtonian fluids. If the viscosity decreases as the stress increases, the fluid is called a psuedo-plastic or stress thinning fluid. Latex paints are designed to have psuedo-plastic behavior- you want the paint to flow easily off the brush when it's moving, but stop flowing once it's on your wall. Another example is toothpaste. Put a small amount of toothpaste on an electric toothbrush. As long as it is turned off, the toothpaste behaves like a solid and does not flow, but as soon as you vibrate the toothbrush the toothpaste will begin to flow and even dance around. Another example of a non-Newtonian fluid is the slime or silly putty we make with Elmer's Glue and borax in one of our other popular activity stations. While the actual behavior is rather complex and difficult to categorize, it will flow fairly easily (i.e. act like a liquid) under low stress, but break (like a solid) if too much stress is applied. Jell-O has somewhat similar behavior.
It is also possible for the viscosity to change with the duration (rather than the magnitude) of the applied stress. If the viscosity continues to decrease over time as a constant stress is applied, the fluid is said to be thixotropic. Ketchup or tomato paste is an example of a thixotropic fluid. Remember the Hines Ketchup commercial (to the soundtrack of Carly Simon's "Anticipation")? To help pour ketchup out of the bottle you should shake or vibrate it quickly, the viscosity soon drops and the ketchup flows more easily.
Another odd but difficult to categorize non-Newtonian fluid is quicksand (fine sand particles suspended in water). When quicksand vibrates (such as during an earthquake) water flows around the sand particles and makes them more buoyant (a process called liquefaction), decreasing the apparent viscosity. When this happens a building or person on the surface can sink into the quicksand. When a person is trapped in quicksand, however, his movements can create local regions where the water flows away and the sand particles are compacted into a solid. Thus the quicksand has a very complicated behavior. To escape from quicksand, simply relax and move very gently and slowly. Because the sand/water fluid is much more dense than your body, you will literally float to the surface over time, rather than sinking to your death as is always portrayed in the movies.
Yet another strange fluid is a mixture of cornstarch in vegetable oil (such as corn or canola oil). Prepare a much less viscous mixture (about 2 parts cornstarch to 1 part oil). It should pour easily from one cup to another with the consistency of pancake batter under normal conditions, but if a strong electric field is present the viscosity will increase dramatically. This is an example of an electro-rheological fluid. To demonstrate this, rub a balloon on your shirt or in your hair to create an electric charge on its surface. As someone slowly pours the fluid from one cup into another, carefully bring the balloon near the pouring stream and the fluid will not only bend towards the balloon, but actually stop flowing and "freeze" in mid-air. Remove the balloon and the stream will begin to flow again immediately. Materials with this property are used in some automotive transmissions.