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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'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: