what you'll need
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.
references and links to more information
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!):
Luz de Dulce - Triboluminiscencia (en español)
Admítelo, te encanta comer dulces. Pero, ¿sería aún mejor si sus dulces pudieran producir sus propios destellos de relámpagos? En este experimento, le mostraremos cómo hacerlo.
Lo que necesitarás
¿El caramelo brilla en la oscuridad antes de aplastarlo? ¿Observó un destello de luz mientras machacaba el caramelo con sus pinzas? No es muy brillante y sucede muy rápido, pero debería ver un destello de luz fría, como una chispa. Si no funciona al principio, inténtelo de nuevo y asegúrese de que la habitación esté oscura y sus ojos estén bien adaptados. ¿De qué color es la luz? ¿Dura mucho? ¿Se calientan los dulces? ¿Frío? Si tienes diferentes tipos de dulces o algunos terrones de azúcar, ¿observas alguna diferencia en la luz de cada uno? Si tienes una menta sin azúcar, ¿también produce un destello de luz? Si tiene otros dulces, pruébelos y haga observaciones.
Qué está pasando
Esto no es un truco de magia o una ilusión óptica, ¡realmente produciste luz a partir de un dulce ordinario! Esta es una demostración de triboluminiscencia, que significa hacer luz frotando, raspando o aplastando algo, que es muy diferente de la mayoría de las formas con las que está familiarizado para hacer luz. Nada se quema, brilla o se calienta como el filamento de una bombilla (aunque puede haber un poco de calor debido a la fricción cuando se tritura el caramelo). Tampoco es una reacción química como las barras de luz con las que quizás hayas jugado antes. Y aunque se parece a las chispas que se ven cuando algunos metales (como el hierro) son rayados o raspados por materiales muy duros (como el pedernal), en realidad se producen por combustión espontánea (quema) del metal como superficies recién cortadas de las piezas muy pequeñas reaccionan con el oxígeno del aire.
Los científicos no comprenden muy bien la triboluminiscencia, pero la mayoría piensa que es similar al rayo que se ve durante las tormentas o durante una de nuestras demostraciones de electricidad fría, que en realidad es un tipo de descarga de plasma. Muchos materiales, como el azúcar de este experimento, tienen una estructura cristalina muy especial que hace que los electrones se desprendan del núcleo de sus átomos. Dado que los electrones tienen una carga eléctrica negativa y los protones en el núcleo tienen una carga positiva, la separación de estas cargas puede crear un campo eléctrico lo suficientemente fuerte como para arrancar electrones de las moléculas de gas en el aire, un proceso llamado ionización, y crea un plasma localizado. Estos iones de plasma pueden chocar contra otras moléculas y transferir energía, lo que hace que emitan fotones de luz. Esos son los destellos que observaste. La mayor parte de la energía luminosa se emite en la parte ultravioleta (UV) del espectro electromagnético, que está justo fuera del rango de luz que es visible para los humanos, pero afortunadamente hay un poco de luz violeta y azul que podemos ver.
Debería haber notado que el destello de luz del Wint-O-Green Life Saver fue significativamente más brillante, y también pudo haber parecido durar un poco más. Esto se debe a que la gaulteria, el aromatizante utilizado en estas mentas, es un tinte fluorescente natural llamado salicilato de metilo. Las moléculas fluorescentes pueden absorber energía de longitudes de onda cortas y emitir su propia luz en una longitud de onda más larga. En nuestro experimento, la gaulteria de la menta absorbe mucha energía ultravioleta que no podemos ver y emite una luz azul verdosa que podemos ver, por lo que nos parece mucho más brillante.
Variaciones y actividades relacionadas
Pruebe este experimento con azúcar de mesa común (que tendrá en abundancia después de triturar todos los terrones de azúcar). Pon unas cuantas cucharaditas en una taza o plato de vidrio transparente (una taza medidora Pyrex funciona muy bien) y muele lentamente el azúcar con una cuchara de metal para observar los destellos de luz azul. Prueba el azúcar en polvo. También puedes hacer el experimento en tu boca (¡muerde con mucho cuidado para no romperte un diente!) Asegúrate de mirarte en un espejo para que puedas ver los fríos destellos de luz en tu boca.
Aquí hay otra forma de demostrar la triboluminiscencia: coloque un trozo de cinta adhesiva en un espejo de vidrio o un plato, luego rómpelo rápidamente en la oscuridad. Si tiene suerte, debería ver un destello de luz, pero en nuestra experiencia, ¡este experimento puede ser muy delicado! Otros también han hecho esto con cinta adhesiva.
Como se mencionó anteriormente, la triboluminiscencia no se comprende completamente. ¡Quizás seas el científico que finalmente explique este fenómeno! La mayoría de los científicos creen que las moléculas de aire juegan un papel importante y que la luz no debería producirse si el azúcar está húmedo o bajo el agua, pero hemos observado destellos de luz en ambos casos. ¿Qué pasa con otros líquidos? La humedad en el aire también puede afectar los resultados (esto es especialmente cierto para el experimento de la cinta adhesiva). Háganos saber lo que observa en sus experimentos en el blog Comentarios a continuación.
referencias y Enlaces a más información
Algunas fotos y videos de destellos de luz triboluminiscente:
Video de YouTube que ensaña la luz usando Duck tape:
Radiografías de cinta adhesiva:
Artículo de Wikipedia sobre triboluminiscencia:
Artículo de revista de investigadores de Towson State University (sí, ¡científicos reales estudian esto!):
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what you'll need
Check out this video from Science Off Center and the St. Louis Science Center
When you run a plastic comb through your hair it pulls lots of very tiny particles called electrons away from your hair. We say these electrons have an electric charge, and when electrons (or other charged particles) move we call this electricity. If lots of extra charged electrons are stuck on the comb and not moving, however, we sometimes call this static electricity. This static electricity creates a force around the comb which can push or pull other charged materials, kind of like a magnet, and it turns out that water molecules are easily pulled by this force. That's why the stream of water from your faucet bends when you hold the charged comb nearby. The closer water is to the comb the stronger the force and the more it moves, but if the comb gets wet it can discharge, allowing those extra electrons to escape, thus there is no force left to pull on the water stream.
more detailed explanation
Everything is made up of matter, matter is made of atoms (which can combine to form molecules), and atoms are made up of protons, neutrons and most importantly for this experiment, electrons. The protons and neutrons are stuck together in the nucleus of the atom, while the much tinier electrons fly around outside the nucleus. It's kind of like the way planets orbit around the sun (the electrons are like the planets and the nucleus is like the sun), but electrons don't follow orderly paths like planets do, it's more like a cloud of electrons (called orbitals) surrounding the nucleus. What keeps electrons from flying away from the nucleus is a force called the electric field force. We say that electrons and protons have an electric charge, or just charge. Electrons have a negative charge (-), protons have a positive charge (+), and charged objects create an electric field that can attract or repel other charged objects (neutrons don't have any charge- they're neutral). Oppositely charged particles attract each other and the closer they are to each other, the stronger the field and the force of attraction. Particles with the same charge (such as two electrons or two protons) repel each other (the closer they are the stronger the field and repulsive force). [BTW- If this reminds you of the way magnets can attract or repel each other depending on their north or south poles and how close they are to each other, you're right. Electric and magnetic forces (and fields) are actually related to each other, so this is really called the electromagnetic force (and electromagnetic field). This is one of the four fundamental forces in nature (see the link below to learn more about this and the other fundamental forces that hold atoms together).]
Different types of atoms (called elements) and the molecules they combine to form may be stronger or weaker when it comes to attracting and holding onto their electrons. In our case, the molecules that make up the plastic comb hold onto their electrons much more strongly than the molecules which make up your hairs hold onto their electrons. When you comb your hair, some of the comb's molecules get very close to some of the hair's molecules- so close, in fact, that the atoms in the comb actually start attracting or pulling on the oppositely charged electrons belonging to the atoms in the hair. Likewise, the hair's atoms also start pulling on the comb's electrons. It's kind of a tug-of-war battle, but with electrons (using the electric field force) instead of rope. Normally most molecules have the same number of negative and positive charges, (so we say they are neutral), but when you take the comb away from your hair some of the hair's electrons are actually pulled away, sticking to the comb's atoms instead (remember that the comb's atoms are stronger at attracting electrons). Thus the comb now has extra electrons- and extra electric charges- and your hairs have too few electrons, so we say the comb is now negatively charged and your hair is now positively charged. That's why your hairs are attracted or stand up if you bring the comb close to your head- because the opposite charges attract each other. The more you comb your hair, the more charges build up, and the stronger the electric field (and attractive force) will be. Charging an object this way is called contact electrification, or the tribo-electric effect (see the reference link below for more detail). This is also commonly called static electricity, but many scientists don't like this name, because electricity is defined as moving electric charges, thus static (non moving) electricity doesn't really make sense.
Water molecules are made up of two hydrogen atoms and one oxygen atom attached to form a triangle shape, kind of like Mickey Mouse's head- oxygen is the face and the two hydrogens are his ears. All of the electrons in these atoms move around in such a way that the oxygen corner of the triangle is more negatively charged and the hydrogen corners are more positively charged. This kind of molecule is called dipolar. Since water molecules in a liquid are free to move around and rotate, when you hold your negatively charged comb near a stream of water, the positive (hydrogen) sides of each water molecule are attracted to the comb's electric field and rotate so that they point towards the comb. This means that the side of the water stream closest to the comb is more positively charged and the opposite side of the stream is more negatively charged. Finally, the electric field of the negatively charged comb attracts (or pulls) on the positively charged side of the water stream, while it also repels (or pushes away) the opposite (negatively charged) side of the stream, but since the positive side is closer, the attractive force there is much stronger and the stream of water is pulled towards the comb. If you move the comb to the other side of the stream the water molecules simply rotate again to point the opposite direction, so that the side of the stream closest to the comb is once again more positive, and thus the stream still bends toward the comb.
By the way- if you hold a magnet near the stream of water nothing happens. It is neither attracted or repelled by the magnet. But that doesn't mean that water (or anything made mostly of water, like frogs or even people) can't be influenced by a magnetic field- you just need a really powerful magnetic field. In fact, if your magnet is strong enough you can even levitate a frog.
Did you notice that if you get the comb wet, it no longer bends the water? The extra electrons that were stuck on the comb are able to move into the water and escape, thus the comb is no longer charged (and no electric field present). Also, if you wet the comb first, then run it through your hair, the water prevents it from grabbing electrons from your hair and charging up, so this doesn't bend a stream of water either. Even a microscopically thin layer of water on the comb, which can happen on a humid day, will prevent it from charging up (if you're having trouble making the experiment work, this may be the problem).
variations and related activities
You can also bend the water stream with a rubber (Latex) balloon or plastic rod (PVC plumbing pipe works very well). Just charge them up by rubbing with your hair, a cotton towel or T-shirt or a piece of fur. Try lots of different materials. This related activity (and video) shows you how to charge a balloon or PVC pipe and make some other objects move. Check out the references below for more information about the tribo-electric series and charging different materials.
Try making the faucet drip instead of a continuous stream of water. Are the droplets attracted to the comb? And while you're in the bathroom anyway, take a small piece of toilet paper and dangle it vertically near your comb. It should also be attracted to the comb, because the comb's electric field induces a small electric dipole moment (like a charge) in the paper's molecules that is attracted to the comb. In fact, if you tear the toilet paper into really tiny pieces you can even use the charged comb to lift them right off a table or countertop, and even make them seem to dance!
Here is a different way to do this experiment:
references and links for more information
Other science website do this experiment:
More on atoms and contact electrification (static electricity):
The tribo-electric effect (contact electrification) and tribo-electric series:
The fundamental forces within an atom:
Induced dipole moments or charges:
Have a question or comment? Let us know at the bottom of the page.
Return to Try Science at Home
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what you'll need
You will also want plenty of space to move around as you levitate objects.
All materials are made up of molecules that are composed of atoms (except for pure elements which are just atoms). Atoms are made of even smaller sub-atomic particles called protons, neutrons and electrons. Protons and neutrons form the nucleus of the atom, while the much, much smaller electrons move around the nucleus. You can sort of picture the nucleus like the sun and the electrons orbiting the nucleus like the planets orbiting the sun (this isn't exactly correct, but it's okay for our simple picture). We say that electrons have a negative electric charge while protons have an opposite or positive charge, and these oppositely charged particles are attracted to each other (like a magnet that sticks to your refrigerator), which is what keeps the electrons in orbit around the nucleus (the neutrons have no electric charge and we can just ignore them for now).
These electrons are so tiny and move around so easily that they can sometimes jump from one atom to another atom in the material, or even from an atom on one object to an atom on a different object. When two objects made of different materials, such as the latex balloon and the cotton or wool cloth, are rubbed together, the protons in each object not only attract their own electrons, but may begin to attract or pull on the electrons in the other object, sort of like a game of tug-of-war. If we now separate these two objects the one made of a material which can attract electrons the strongest may actually steal many electrons from the weaker material. This gives the "winning" object a lot of extra electrons, and thus a lot of excess negative electric charge, while the other object is left with the opposite or excess positive charge. This process is called contact electrification or triboelectricity, although it's more commonly referred to as static electricity. There are triboelectric series charts (see the reference links below) showing which materials can 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.
To explain how electrically charged objects (or individual protons and electrons) attract each other scientists say that they create an invisible electric field or force in the space around them. This electric field can not only attract an object or particle with an opposite electric charge (like a proton attracting an electron), but can also repel a particle if it has the same charge, i.e. two electrons will repel each other. Thus similarly charged objects (either 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 when you rubbed a balloon on your hair. Rubbing objects together just brings more of their surfaces close enough 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 and they attract each other (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 which one is positive and which is negative, but as long as 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 charged balloon or PVC wand produces a repelling or pushing force on the hoop. The closer you bring it the stronger that force will be, so that the hoop stops falling and rises up instead. If you hold the balloon or wand at just the right distance below the hoop you should be able to make it levitate almost motionless- bring it closer and the hoop rises, farther away and the hoop falls. Move your wand or balloon around and you can make the hoop dance and perform tricks!
Different materials are classified as conductors if their electrons can move around easily through the material, or insulators if their electrons cannot move easily. Latex rubber, plastic and Styrofoam are insulators, while metals such as copper or the aluminum in a soda can are conductors. When we charge insulating materials like the Latex balloon by rubbing it, the separated charges stay put for a long time since their electrons can't easily move around, and that's why it remains charged. Metals are very different however, which is why electric wires and cables are made with metal conductors so that electricity (moving electrons) can flow more easily. These electric wires are covered with rubber or plastic insulation so we don't get a shock when we touch them.
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 on 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 , try cutting off 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, but plastic tables do not.
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
You can also use the electric field force from a charged balloon or PVC wand to move other objects. Lay an empty aluminum soda can on its side, charge up your balloon or PVC wand then hold it very close to the can without touching it. The can should begin to roll towards your wand, so be sure to move it away as the can rolls to keep from touching. Place your wand on the other side of the can and you can make it stop, then begin rolling the other direction. Notice that you didn't need to charge the can for this to happen! That's because metals like aluminum are conductors, unlike the PVC plastic or Latex balloon. When we bring an aluminum can or a metal spoon near a charged object such as a balloon, the electrons in the metal can move around easily. If the balloon has a negative charge, electrons in the soda can (which also have negative charge) are repelled (remember similar charges repel each other) and move as far away from the balloon as possible (i.e. to the back side of the can), leaving fewer electrons and thus a positive charge on the side of the can nearest the balloon. 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 a balloon; if a 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 soda can), so the balloon should also move towards your fingers or body. That's also why the polyethylene hoop likes to stick to your body!
Have you ever noticed little bits of Styrofoam (like packing "peanuts") clinging to your hand or other objects? Do clothes cling to you or other clothes when you take them our of the dryer (i.e. "static cling")? Styrofoam peanuts (and clothes) can pick up an electric charge very easily by bumping into other peanuts (or clothes) or just about anything else, and thus will be attracted strongly to other objects- which you've probably noticed if you ever tried to pick them up! Try moving them around with your charged balloon or wand. The tiny beads inside a beanbag chair are also made of Styrofoam, so you if you have some you can put them inside a dry, empty plastic soda bottle and screw on the cap (if you don't have an old beanbag chair you can tear some packing peanuts or other pieces of Styrofoam into tiny bits instead). Now rub the outside of the bottle on your cotton shirt or hair and shake the bottle to transfer electric charge to the bits inside, then move a charged balloon or PVC wand near the bottle to see what happens! Also try moving a metal spoon, fork (or your aluminum soda can) near the plastic bottle filled with Styrofoam bits. Just as when you rolled an aluminum soda can above, electrons in the metal can move around due to the influence of the electric field from a charged object. Electric fields around conductors are 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.
If you rub a balloon on your cotton shirt or hair in a very dark room you may see sparks and hear crackling! The sparks are actually tiny lightning bolts, and the crackling tiny thunder claps! The electric field around your charged balloon is strong enough to move or even rip electrons from nearby air molecules, and the energy released as they crash into other air molecules heats the air to make the sparks and crackles. This is also what happens when air moves rapidly in clouds during a storm to make the really big lightning and thunder, and when you rub your feet on carpet and touch a doorknob or "shock" a friend. You might also see sparks at night when you rub against your blankets in bed.
We can even use the force of an electric field from a charged object to bend water! Adjust the faucet in your bathroom or kitchen sink so that a very thin continuous stream of water flows (rapid dripping also works). Charge up a PVC wand, balloon or even run a plastic comb through your hair then carefully hold it close to the stream of water. Make sure your charged object doesn't get wet, however, as this will discharge it (electrons can move through water- that's why you never drop a toaster or hair dryer in the bathtub!) If this happens dry the PVC or balloon completely, then charge it up and try again. The stream moves because water molecules are dipolar, i.e. they have negatively and positively charged ends.
There are even more electric charge activities and experiments in the links below.
references and links to more information
More about triboelectricity, contact electrification and the Triboelectric Series:
Measure electric charge with a DIY electroscope:
Fun with static electricity videos:
More static electricity activities:
Just for fun, levitating a frog in a very strong magnetic field:
Have a question or comment? Let us know at the bottom of the page.
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: