Soap Boat Racers
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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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