what you'll need
You'll also want a large washable table or drop cloth if you're working inside, as this can be a pretty messy experiment. Towels and a bucket of clean water are a good idea.
prepare your oobleck
Preparing a cornstarch and water Oobleck mixture.
There are three main states of matter- solids, liquids and gasses (there is also a fourth state called plasma, and even more states that you may learn about if you study science in college). These three states have very different physical properties, so it is usually very easy to tell them apart. We know that solids keep their shape and are often hard, liquids take the shape of their container but can pour and flow, and gasses can expand to fill all available space. Scientists have created another category (although it's not a true state of matter) called a fluid which includes any substance that will pour or flow if pushed. Liquids like water are obviously fluids, but a bucket of sand or rocks (which are solids) can also be fluids, as can mixtures of solids and liquids, and even gasses.
But what if a material has properties that fit with more than one state of matter at the same time? We're not talking about cooling a liquid until it freezes and becomes solid, or boiling it until turns to gas- all substances act this way depending on their temperature. Oobleck can act like a liquid- it takes the shape of its container, it's easy to stir and you can pour it- but if you push it too hard or too quickly it instantly acts like a solid- it stops flowing, keeps its shape and can even break. So is Oobleck a solid or a liquid? The answer is both- and neither. Confusing, right? It turns out that not everything easily fits into just one state of matter category. Oobleck is a fluid because it can flow, but it's certainly not like water or other normal fluids, so we call this a Non-Newtonian fluid (named after the famous scientist Sir Isaac Newton). Scientists aren't exactly sure why Oobleck acts this way, but the full explanation below will help you understand what might be happening. There are many other common substances (like ketchup or toothpaste) that also have some strange properties if you study them closely. You can find out more about these in the related activities and links below.
More detailed explanation
When a large amount of cornstarch is added to a smaller amount of water it does not dissolve to make a liquid solution. It can flow, however, so we refer to this mixture as a fluid. 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 (i.e. a sideways push or 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 gravity. The term fluid and liquid are often used as synonyms, but a fluid can actually be liquid, gas, mixtures of either or both, mixtures of solids in a liquid (called suspensions or colloids), or even mixtures of two or more solids. Examples include sand, rocks, honey and maple syrup. If it flows as a shear stress is applied- even 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 (or the more you tip a cup) the faster it flows. 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 honey). 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 first studied them). Oobleck, however, is an example of a (very) non-Newtonian fluid. Its flow rate decreases dramatically (i.e. the viscosity increases dramatically) as the sheer stress increases. The harder and faster you push 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 (check out the video links below), 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 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. 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. There is so much more cornstarch than water in the mixture that there is just no room for the cornstarch molecules to quickly move. If the mixture is made with a much higher proportion of water, however, this behavior is not observed, even though the fluid may still be thicker and more viscous. This is what happens when you use cornstarch to make gravy or pudding.
Oobleck Cornstarch Monsters (find out what's happening in the Related Activities below)
variations and related activities
One of the most interesting things you can do with Oobleck is to vibrate it (move back and forth) very quickly, typically by placing it in a speaker cone and driving it with a frequency generator and amplifier (as shown in the video above). Once we start the amplifier the cone vibrates back and forth between 50-90 Hz (cycles per second) which pushes on the Oobleck as the speaker moves forward, causing it to solidify instantly. As the cone stops and then reverses direction the Oobleck can relax and become liquid-like once again. These solid to liquid transitions occur so quickly that the Oobleck is able to build solid-like fingers and other shapes that seem to grow and come to life right out of the liquid. Some even break free to hop and dance around. Watch the behavior change in the video as we vary the frequency and amplitude of the vibration.
There are many other interesting and very 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 (as well as some nail polish and cosmetics) 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, like we make with Elmer's Glue and borax in one of our other popular activities. 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 (even if it's relatively small in magnitude), the fluid is said to be thixotropic. Ketchup or tomato paste is an example of a thixotropic fluid [actually ketchup is even more complicated, exhibiting both thixotropic and pseudo-plastic properties]. 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 (see the video link below).
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 or someone stuck squirms around) 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 easily 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 actually 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 trapped electric charges on its surface (sometimes called static electricity). As a partner slowly pours the cornstarch-oil fluid from one cup into another, carefully bring your charged balloon near the pouring stream and the fluid will not only bend towards the balloon, but will 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 being used in some automotive transmissions, clutches, brakes and shock absorbers.
references and links to more information
More on Non-Newtonian fluids [Note that as you read more about Non-Newtonian fluids you may find a lot of contradictory information. This is because the behavior of these materials are often very complex and difficult to categorize]:
More activities and videos with Oobleck:
Running across a pool filled with Oobleck:
More Oobleck Cornstarch Monsters:
Why is Ketchup so hard to get out of its bottle?:
Electro-rheological and magneto-rheological fluids:
What you'll need:
You will also want plenty of space to move around as you levitate objects.
All materials are made of molecules that are composed of atoms (except for pure elements which are just atoms), which in turn are made of protons, neutrons and electrons. The electrons, which have a negative electric charge, are the smallest and most mobile of these sub-atomic particles and can easily move from one atom to another, even from an atom on one material to an atom on a different material. When two different materials, such as the latex balloon and the cotton or wool cloth, are brought very close to each other, the atoms in each material begin to tug at the electrons on the other, and the material which holds onto electrons the strongest may actually steal many of them from the weaker material once they are separated again. This gives the stronger material a lot of excess electrons, and thus a lot of excess negative electric charge, while the other material is left with an opposite or excess positive charge. This process is properly called contact electrification or triboelectricity, although it's more commonly referred to as static electricity. We can find triboelectric charts showing which materials steal electrons from other materials, i.e. which will become negatively or positively charged upon contact with another material in the chart, but triboelectricity can be finicky, so it's not always clear which way the electrons will go in all cases, and sometimes the behavior just doesn't agree with the chart at all.
Electrically charged materials create an invisible electric field or force in the space around them, which can attract or repel other charged objects (or electrons). Similarly charged objects (positive and positive or negative and negative) repel each other, while oppositely charged objects attract each other, sort of like the attractive and repulsive forces of magnets (in fact, magnetism is just another form of exactly the same fundamental force observed here- but that's for another activity). This is what's happening when you press the polyethylene plastic hoop against the tabletop surface, or rub the balloon with the cotton cloth. You have probably also noticed this if you ever rubbed a balloon on your hair. Rubbing them together just brings more of their surfaces closer together so that the atoms in one material can tug at the electrons of the other (it's not due to friction, so you don't need to rub hard). When you pull them apart you each will have opposite electric charge: the plastic hoop and the tabletop have opposite charge, i.e. one is positive and the other negative (that's why they cling or stick together); and the ballon or PVC and the cloth also have opposite charge, one positive the other negative. It can be very difficult to tell whether any particular object has a positive or a negative charge, but if the plastic hoop has the same charge as the balloon or PVC wand, whether it's actually positive or negative, they will repel each other (of course if they are oppositely charged they will instead attract and perhaps even stick to each other).
When you toss the plastic hoop in the air it starts to fall due to the pulling force of gravity, but your balloon or PVC wand can produce a pushing force on the hoop, and the closer you bring it the stronger that force will be. If you hold the balloon or wand at just the right distance below the hoop you should be able to make it levitate almost motionlessly- bring it closer and the hoop will rise, farther away and the hoop will fall.
Materials can also be classified as conductors if their electrons can move around easily through the material, or insulators if their electrons cannot move easily. Latex and Styrofoam are insulators, while metals such as the aluminum can are conductors. Thus when we charge the balloon by rubbing it, the charges stay put for a long time since their electrons can't easily move around. Metals are very different however. When we bring an aluminum can or a metal spoon near a charged object such as a balloon, electrons in the metal can move around easily. If the balloon has a negative charge, electrons in the pop can (which also have negative charge) move as far away from the balloon as they can, leaving a positive charge on the side of the can nearest the balloon, and since opposite charges attract each other, the can will move towards the balloon. The same thing happens when a metal spoon is held close to the balloon; if the balloon is hanging from a string and free to move, it will be attracted to the spoon. Your fingers are also conductors (though not as good as the aluminum in the pop can), so the balloon will also move towards your fingers or body. When a balloon filled with Styrofoam bits is charged (let's say it's negative), the bits will touch the balloon and acquire the opposite charge (positive) and stick to the wall of the balloon. Now when a metal spoon (or your fingers) is held close to the balloon, the side of the spoon facing the balloon becomes positively charged, and this positive charge (or the electric field arising from the positive charge) is strong enough to repel the Styrofoam bits (because they're are also positively charged), thus they jump way very quickly. Electric fields around conductors are also much stronger near sharp points and edges, thus as you turn the spoon (or point your fingertips), the Styrofoam bits will experience stronger forces and move more quickly.
Troubleshooting - What Can Go Wrong
Contact electrification or triboelectricity can be very finicky, often due to various types of contamination or environmental factors which can influence the process. Thus you might have trouble charging your objects, or even get the opposite of what you expect to see. Some days it just doesn't want to work at all, especially if it is a very humid day. Moisture in the air can deposit a thin layer of water molecules on some objects which allows the static electric charges to move away. Oil from your hands can also contaminate objects in this way, so if your plastic hoop doesn't seem to work , cut a fresh one.
In our experience rubbing a Latex balloon or PVC pipe with cotton cloth and rubbing a polyethylene bag on a wooden surface should produce the best results, but if these combinations don't seem to work for you, try something different. You can substitute wool, fur or polyester fabric for cotton, and you can try different wooden surfaces (painted, varnished, waxed, bare wood, etc.). Some kitchen countertops (like Formica) may also work very well.
In any case, the more you practice the better you will get at levitation, so keep experimenting. And who knows- a little Wingardium Leviosa probably couldn't hurt either!
Variations and Related Activities:
References and links to more information:
Admit it- you love to eat candy. But would it be even cooler if your candy could produce its own flashes of lightning? In this experiment we'll show you how to do just that.
Also be sure to ask your mom, dad or another adult to help- they'll want to watch anyway when you tell them how cool this experiment is!
Does the candy glow in the dark before you crush it? Did you observe a flash of light as you crunched the candy with your pliers? It's not very bright, and it happens very quickly, but you should see a cool flash of light, like a spark. If it doesn't work at first, try again, and make sure the room is dark and your eyes are adjusted. What color is the light? Does it last very long? Does the candy get hot? Cold? If you have different types of candy or some sugar cubes, do you observe any differences in the light from each? If you have a sugar-free mint, does it make a flash of light too? If you have any other candies, try those and make observations.
This is not a magic trick or an optical illusion- you really did produce light from an ordinary piece of candy!. This is a demonstration of tribo-luminescence, which means to make light by rubbing, scratching or crushing something, which is very different from most of the ways you are familiar with to make light. Nothing is burning, glowing or getting hot like the filament in a light bulb (although there may be a little heat from the friction as the candy is crushed). It's also not a chemical reaction like the light-sticks you may have played with before. And though it does look sort of like the sparks you see when some metals (like iron) are scratched or scraped by very hard materials (like flint), those are actually produced by spontaneous combustion (burning) of the metal as freshly cut surfaces of very small pieces react with oxygen from the air.
Triboluminescence is not very well understood by scientists, but most think it is similar to the lightning you see during storms or during one of our cool electricity demonstrations, which is actually a type of plasma discharge. Many materials, like the sugar in this experiment, have a very special crystalline structure that causes electrons to be ripped away from the nucleus of their atoms. Since electrons have a negative electrical charge and the protons in the nucleus have a positive charge, separating these charges can create an electrical field strong enough to rip electrons off gas molecules in the air, a process called ionization, and creates a localized plasma. These plasma ions can then slam into other molecules and transfer energy which causes them to emit photons of light. Those are the flashes you observed. Most of the light energy is emitted in the ultraviolet (UV) part of the electromagnetic spectrum, which is just outside the range of light that is visible to humans, but fortunately there is a bit of violet and blue light that we can see.
You should have noticed that the flash of light from the Wint-O-Green Life Saver was significantly brighter, and may have also seemed to last a little longer. This is because wintergreen, the flavoring used in these mints, is a natural fluorescent dye called methyl salicylate. Fluorescent molecules can absorb energy of short wavelengths and emit their own light at a longer wavelength. In our experiment the wintergreen in the mint absorbs a lot of UV energy that we can't see, and emits greenish-blue light which we can see, so it appears much brighter to us.
Variations and Related Activities:
Try this experiment with ordinary table sugar (which you will have plenty of after you crush all your sugar cubes). Put a few teaspoons into a clear glass cup or dish (a Pyrex measuring cup works great) and slowly grind the sugar with a metal spoon to observe flashes of blue light. Try powdered sugar. You can also do the experiment in your mouth (bite very carefully so you don't break a tooth!) Be sure to look in a mirror so you can see the cool flashes of light in your mouth.
Here's another way to demonstrate triboluminescence: place a piece of Scotch tape onto a glass mirror or plate, then quickly rip it off in the dark. If you're lucky you should see a flash of light, but in our experience this experiment can be very finicky! Others have done this with Duct tape too.
As was mentioned earlier, triboluminescence is not fully understood. Maybe you will be the scientist to finally explain this phenomenon! Most scientists believe air molecules play a role and that light should not be produced if the sugar is wet or under water, but we have observed flashes of light in both cases. What about other liquids? Humidity in the air can also affect the results (this is especially true for the scotch tape experiment). Let us know what you observe in your experiments in the Comments blog below.
Links to more information and activities:
Some photos and videos of triboluminescence light flashes:
Others instructions for this experiment:
Youtube video showing flashes of light from Duct tape:
X-rays from Scotch tape:
Wikipedia article on triboluminescence:
Journal paper from Towson State University researchers (yes, real scientists study this!):