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This experiment is all about what's happening on the surface of the water. It's tempting to say that the little bread clip boat is floating in the water, but it's actually held up by water molecules at the surface pulling on other water molecules around them (cohesion) to create a strong force called surface tension. It's almost as if they form an invisible film that covers the entire surface, and it's strong enough to keep your boat from sinking! You can prove that your boat can't float on it's own by pushing it into the water- it will sink to the bottom of the pan.
When you add a drop of dish soap to the water soap molecules begin to spread out like a film or puddle, taking the place of water molecules at the surface. This is because soap is type of chemical called a surfactant, and its molecules have a much lower or weaker surface tension, i.e. they can't pull other molecules around them as strongly as the water molecules do. Imagine a tug-of-war competition between two teams- the soap team and the water team. The water team is much stronger, so they win, pulling the soap molecules back with them and stretching the soap film out across the surface. This process of a liquid with a higher surface tension pulling a liquid with a lower surface tension is called the Marangoni effect (the reference link below has a nice video explaining what is happening).
When you add a drop of dish soap to the water inside the hole of your boat it lowers the surface tension inside and the water molecules behind your boat pull soap out of the hole through the little gap (like a rocket nozzle). The force of these soap molecules moving backwards produces an equal and opposite force on the boat, moving it forward. This is an example of Newton's Third Law of Motion (see the reference links below to learn more about Sir Isaac Newton's famous Laws of Motion).
Because the surface tension of water is so much stronger than that of soap, the water quickly stretches out and expands the soap film until it covers the entire surface of your pan. Once this happens there is no more force to propel your boat and it stops. Even a tiny drop of soap contains more than enough molecules to cover the entire surface (the film can actually keep stretching until it is only a single molecule thick!), so if you want to race your boat again you must remove all of the soap from the surface by dumping out the water and rinsing everything.
Variations and related activities
There are more fun experiments that demonstrate the cohesive force or surface tension of water. First, place a glass or plastic cup (it should have a smooth rim) in your empty pan (just to catch any water that spills), then fill the cup with water completely to the top (just before it spills over). Using a spoon (or a pipette if you have one) carefully add more water a few drops at a time until the surface of the water bulges over the top of the glass without spilling. You can really see just how strong the surface tension is as it keeps the water from spilling over the side (if it does spill over the side you probably added the extra water too quickly; try again). Now that you are impressed by the surface tension of pure water, add a single drop of dish soap to the water in the cup (or touch it with your soapy Q-tip or toothpick). The water should instantly spill over the side as the soap lowers the surface tension, making it too weak to keep bulging above the top of the cup.
Another fun experiment is to see what else you can "float" on the water surface. [Note- we'll say "float" here, but what we're really doing is suspending objects on the surface of water due to surface tension. An object only truly floats in a fluid when its density is less than that of the fluid, and the objects we'll be using are more dense than water, so they would sink if not for the surface tension.] Again place your glass or plastic cup in the empty pan and fill it to the top with clean water. Try to place a paper clip on the surface of the water without it sinking. This can be a bit tricky, but keep trying. Some tips that might help: use smaller paper clips, bigger ones may just be too heavy for surface tension to hold; first balance the paper clip like a teeter-totter on the edge of the cup, then gently nudge it onto the water surface; another trick that might help is to first float a small piece of paper towel or napkin on the surface, lay the paper clip on the paper, then gently sink the paper using another paper clip or toothpick. See if you can "float" small buttons or thumb tacks the same way (make sure you use the type of thumb tacks shaped like little umbrellas, and larger or plastic-coated ones usually work better). Once you have some objects suspended on the surface, add a drop of dish soap again and watch what happens. [Note that some buttons may still float even after you add the soap- if they are only slightly more dense than water the surface tension of the soap layer may be strong enough to hold them up.]
You can indirectly observe a soap film spreading across the water. Add some clean water to your pan then sprinkle some pepper flakes all over the surface. Add a drop of dish soap near the middle of the pan and watch what happens. Since the pepper flakes are floating on the water they will move as it pulls away, allowing you to actually measure the size of the soap film. You can also add pepper flakes to the water before you launch your boat to better observe what is happening when you add the soap.
Try various sizes and shapes of bread bag clips, and trim them to make different boat shapes. You can also cut boats from old playing cards or other paper that has a waterproof coating, thin Styrofoam sheets, plastic or other materials. Try various shapes for the "fuel" hole and "nozzle" channel. How does this affect the performance of your boat? Trying racing boats with your friends. Remember that you will need to dump the soapy water and refill the pan with clean water before each race. Could you design a boat that that will always move in a circle, or just spin without really going anywhere (see the science4fun reference link below)?
Instead of adding the soap to the hole inside the bread clip while it is in the water try placing a small drop of soap directly on the boat first (somewhere around the fuel hole or the nozzle port), then placing the boat in the water. You can also try touching your soapy toothpick behind the boat once it's in the water rather than inside the fuel hole. Does it still move? Does it go as fast or as far as before? When you add the soap behind the boat the surface tension of the water will try to pull water and soap into the boat hole which- according to Newton's 3rd Law- should make the boat move backwards, but it still goes forward instead. How can that be? In this case something different is happening. As the soap film forms in back of the boat the water behind the soap pulls backwards, but the water in front of the boat is pulling forward, and this drags the boat along with it (sprinkle pepper in the water to observe this). This dragging effect also contributes some of the force to move the boat even when you do add the soap inside the hole.
Finally, soap is not the only liquid with a lower surface tension than water. Try adding a small drop of rubbing alcohol (isopropyl alcohol) to the fuel hole in your boat instead of dish soap (you will need to use a pipette or small straw). Your boat might not move any faster than it did with soap- at first- but if you keep adding more drops of alcohol your boat will keep going, and going, and going! It doesn't "run out of gas" the way it did with soap. This is because rather than forming a film on the surface (as a surfactant like soap does), alcohol is miscible in water, i.e. alcohol mixes or dissolves completely into the water, allowing the surface tension to recover fairly quickly so that the next drop is just as effective as the last. Theoretically your boat could keep going until you have added almost as much alcohol as the water you started with. This reference link below explains what's happening and describes some other liquids that you can experiment with as alternative "fuels" to better propel your boat.
references and links for more information
Other's versions of this activity:
Surface tension and cohesion force:
More surface tension and cohesion experiments:
Nice video explaining the Marangoni effect:
An even better "fuel" to power your little boat and a great way to make this activity into a real experiment:
Newton's Laws of Motion:
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What you need
If an object moves back and forth repeatedly we say that it is vibrating, and the number of times the object moves back and forth each second is called the frequency of the vibration, which is measured in Hertz (Hz). If it moves back and forth many times each second it has a high frequency, fewer times per second is a lower frequency. Lots of metal objects (like your spoon) will vibrate if you strike or tap them, but they don't just vibrate with any random frequency. Any object has a set of special frequencies at which it naturally prefers to vibrate, called its natural or resonant frequencies, which depend on the various dimensions of the object (size, shape, thickness, weight, etc.) as well as the material from which it is made.
When you tap your spoon it vibrates, and as the spoon moves back and forth it collides with air molecules nearby, causing them to move. Those air molecules in turn collide with their neighboring air molecules, and so on, launching waves of molecules through the air- just like the waves of water molecules you launch when you move your hand back and forth in the bathtub. You may not be able to see the air waves, but you can feel them if you move your hand back and forth quickly near your face! You can also hear these waves (as long as the frequency of the waves is not too high or too low), so we call them sound waves. The frequency of a sound wave corresponds to the pitch or musical note, and our human ears can hear sound waves with frequencies between about 20 Hz (a very low note) and 20,000 Hz (or 20 kHz, a very high note). Of course you would need to move your hand pretty fast to actually hear those sound waves, but you can hear the sound waves traveling through the air when your spoon vibrates, because it's vibrating at a much higher frequency. [And spoons will vibrate at different natural frequencies depending on their size, shape and material- see the related activity below.] Of course we hear all of this because the sound waves travel through the air to your ears, where the vibrating air molecules collide with your ear drum and cause it to vibrate. This finally sends signals your brain which it interprets as sound from the spoon (see the link below to learn exactly how your ears work).
Sound waves don't just travel through air (or other gasses), they can also travel through liquids and even solids in exactly the same way. Just as a vibrating object collides with nearby air molecules to make them vibrate (i.e. they're now moving back and forth), molecules and atoms in liquids and solids can be made to vibrate as well. When you tap the spoon it vibrates, but that also makes the string attached to it vibrate (just like a guitar string), and because the string is wrapped around your finger it vibrates too. If you put your finger in your ear that even makes your skull vibrate a little, which finally makes your eardrum vibrate and you finally hear the sound that came from the spoon again. The sound waves travelled through the string and through your body to your ears. But why does it sound so different this time?
First we must understand that the spoon isn't vibrating with only one single frequency, but rather a series of related frequencies called the harmonic series, harmonics or sometimes the overtones, where each member in the series is a multiple of the fundamental or lowest frequency. For example, if the fundamental frequency (or 1st harmonic) is 500 Hz, then the second harmonic would be 1,000 Hz (2 x 500), the third harmonic 1,500 Hz (3 x 500), etc. When the spoon vibrates some of these frequencies are stronger (i.e. they move more) while others are weaker. The sound we hear from the spoon is really a simultaneous combination of the sound waves from each of these harmonic vibrations, the stronger ones are louder and the weaker ones are quieter (and some perhaps missing altogether). In music this is called timbre, and is the reason that musical instruments or singer's voices may sound very different even when they are playing (or singing) the exact same frequency or note. Finally, which harmonics we hear depends not only only those that are present to begin with, but also on which ones actually make it to our ears.
In the first experiment when you tapped the spoon while you were just holding it, the sound waves you heard only travelled through the air to your ears. In the second experiment you plugged your ears with your fingers, so you couldn't hear most of the sound waves from the air, but you could hear the sound waves- or the vibrations- that travelled through your body, and those vibrations had very different frequencies which sounded deeper and richer, more like a gong or church bell rather than the ordinary spoon in air. Remember we said that objects prefer to vibrate at their own natural frequencies- the string and the bones in your body are very different from air molecules- so they vibrate very differently, and that affects which sound waves travel the best and which of the harmonic vibrations of the spoon will be louder or quieter once they reach your ears. Lower frequencies travel through the string and especially your body much better than higher ones, which is why the spoon sounds so much deeper and richer the second time.
variations and related activities
Vibrating other objects, they sound different because they have different natural frequencies.
Why does your voice sound different to you (but not others) on recordings?
Why does your voice sound different when you breathe helium?
References and links to more information
There's no sound in space:
Human ears and hearing:
Why does your voice sound higher when you breathe helium?:
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what you need
practice your skills first
When you push on something it moves forward, even if only a very tiny bit, and when you let go it relaxes and moves back. If you do this very quickly over and over again, the object moves back and forth continuously. We say the object is vibrating, and that's what the glass is doing as you rub back and forth- it's vibrating. You don't see it move, however, because only the rim of the glass is moving a very tiny distance, and because it's actually moving much, much faster than your finger is (to understand why this is so you can read the more detailed explanation below).
Now you have surely made water waves in the bath tub by moving your hand back and forth (you were vibrating your hand). If the walls of your bath tub were thin and flexible enough to move easily, you could even make waves in the water by pushing or tapping on the sides of the tub instead. When the wine glass vibrates while it's full of water it also makes waves (you can also tap on the sides of the glass to see them), and if these waves are big enough they can even splash water droplets right out of the glass!
more detailed explanation
Objects can be made to vibrate by forcing them to move back and forth quickly. A dry finger will stick to the rim of a wine glass pretty well, but when you wet your finger a little, it will begin to slide. Either way, when you rub the rim of the wine glass in just the right way, your finger will stick for a short time, then slide a little, then stick again, then slide again, etc. (the same thing happens as you rub the handles of the spouting bowl with your wet hands). This is called "stick-slip" friction, and in a sense it's like tapping on the rim of the glass (or bowl) very quickly. This makes the glass begin to move back and forth or vibrate, literally bulging in and out at various places around the rim. The number of times the rim vibrates back and forth per second is called the frequency, and many different vibration frequencies are excited as you rub the rim of the glass. These movements are much to small in magnitude and much too fast to see with your eyes, but we can easily see the waves that are created by the rim pushing on the water in the glass.
Since these vibrations are so small, most of them lose their energy and die out very quickly. Any solid object, however, has a set of special frequencies (and shapes) at which it prefers to vibrate, called its natural or resonant frequencies (and modal shapes). At these resonant frequencies it takes only a very small amount of input motion or energy to produce very large vibrations and large output energies. As you start the wine glass or spouting bowl vibrating with your stick-slip motion, these resonant frequencies are also excited, but since they require only a small input energy to produce large output vibrations, they quickly dominate the motion and last much longer. This is called resonance, and we say the object is resonating. The sound you hear is produced by the resonant vibration of the glass or bowl.
Now back to the waves in the water. The large vibrations along the rim of the glass push on the water, sending waves traveling across the surface. When the waves hit the other side of the glass they bounce back (reflections) and run into other waves traveling in the opposite direction. All of these waves, which are being launched at precise time intervals, begin to combine. In places where two or more wave crests or high points meet, the combined wave will be even higher. Similarly, in places where two or more wave troughs or low points meet, the combined wave will be even lower. In other places crests and troughs from different waves will meet and cancel each other. This creates what are called standing wave patterns (i.e. the combined wave pattern appears to stand still) on the surface of the water. FInally, because the standing wave patterns are created by large resonant vibrations of the glass (or bowl), the standing water waves become very large also, eventually splashing water high into the air. The locations where the water splashes highest corresponds to locations where the glass (or bowl) is moving the most, called anti-nodes. Halfway between each anti-node is a node, a location where the rim of the glass (or bowl) is not moving at all. Near these nodes the standing waves are very small and no splashing occurs. Since the position of the handles are fixed on the spouting bowl, the positions of the nodes and anti-nodes are also fixed, and the water always splashes in the same locations, For the wine glass, however, your finger is moving around the rim, thus the vibrations of the rim as well as the standing wave patterns in the water also move with your finger.
variations and related activities
Just as the moving glass strikes water molecules inside the glass producing water waves, it also strikes air molecules to produce similar waves that travel through the air. We can't see those, but we do hear them as sound waves, and since the glass vibrates most at its resonant frequencies, those are the frequencies of sound (or musical notes) that we hear when it rings. These resonant or natural frequencies of your glass depend on its dimensions and the type of glass from which its made [you might experiment with wine glasses of different sizes and shapes], but as you may have noticed as you performed the experiment, it's actually very easy to change the resonant frequency of any glass- just add water! To demonstrate this, listen carefully to the note as you make the empty glass ring. Next fill the glass about half full of water and ring it again. The note you hear now should be much lower in pitch, because the glass is vibrating with a lower frequency. The mass of the water in the glass makes it heavier and causes it to vibrate more slowly. Experiment with different amounts of water to see how the note it makes as it rings changes. If you have several wine glasses, each with a different amount of water, you can even make a simple musical instrument (sometimes called a glass harp) to play a song (see the video link below). Ben Franklin actually invented a musical instrument based on this that he called the Glass Armonica.
You may not be able to directly see the wine glass move as it vibrates, but there are other ways to prove that it's moving. Sprinkle a few drops of water on the outside of the glass near the top then wet your finger and rub the rim of the glass until it rings as you did before. You should observe the droplets vibrating, showing that the glass is moving. Another way is to place a drinking straw or even a pencil inside the (empty) glass, leaning against the rim. Again wet your finger and rub the rim of the glass until it rings and the straw will begin to bounce around as the vibrating glass strikes it. As this continues the straw might actually stop moving for a moment and remain in the same location even though the glass is still vibrating (which you know because it is still ringing). This is because the rim of the glass moves more in some places and less in others, and the straw has happened to land in a spot on the rim where it is not moving enough to bounce he straw. In fact, if you could rub the glass without moving your finger around (of course that's not really possible, but bear with us), you would find that there are at least 4 locations along the rim where it is not moving at all (called nodes), 4 other locations halfway between each pair of nodes where the rim is moving the maximum amount (called anti-nodes), and at all other locations the amount of movement (or displacement) falls somewhere between the maximum and minimum. The shapes or patterns that the glass makes as it vibrates are called modes, and each frequency your hear corresponds to a different mode of vibration. One of the video links below shows the vibration mode of a wine glass driven by sound waves very nicely. This also demonstrates that just as a vibrating glass generates sound waves in the air, sound waves in the air from another source (in this case a loudspeaker) can actually strike an initially motionless glass and cause it to begin vibrating at the same frequency. If the sound waves are loud enough the glass may even vibrate too much and shatter!
Singing Bowls (coming soon).
references and links to more information
Resonance with wine glasses and singing bowls:
Vibrating a wine glass with sound waves:
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!):