- The physics of skydiving
- The physics of invisibility cloaks
- The science of bouncing
- How do ships float?
- Thomas Young's double slit experiment
- Newton's prism experiment
- Bridge design and destruction! (part 1)
- Bridge design and destruction! (part 2)
- Shifts in equilibrium
- The Marangoni effect: How to make a soap propelled boat!
- The invention of the battery
- The forces on an airplane
- Bouncing droplets: Superhydrophobic and superhydrophilic surfaces
- A crash course on indoor flying robots
- Heat transfer
The differences of surface tension between soap and water can really make things go! This video shows some interesting demos illustrating the Marangoni Effect. Two liquids with different surface tensions will result in a surface tension gradient, resulting in a net force. This phenomena applies to each of the demonstrations shown. Watch a demonstration of how Marangoni stress can be used to propel small boats around (the "soap-powered boat"). License: Creative Commons BY-NC-SA More information at http://k12videos.mit.edu/terms-conditions. Created by MIT+K12.
Want to join the conversation?
- Could we use ecological soaps to propel real boats and stop polluting so much the atmosphere?(73 votes)
- It's an interesting idea, but I don't think it could work. Surface tension is a pretty weak force on a large scale. It's enough to curve the surface of a droplet of water and to move a paper boat, but it's essentially irrelevant at the scale of a real boat.
In addition, in order to propel a boat forward, the amount of soap moving in the opposite direction would be huge and all that soap would have to be stored on board the ship, which would make it heavier, meaning even more soap was required to propel it and so on. I think you'd quickly reach the point where your boat was too big to move.(67 votes)
- Who is the Maragoni that this effect is named for and what did he do in physics?(13 votes)
- i think it is okay to post from wikipedia, since that is meant to be freely available and it is for reference purposes(2 votes)
- what kind of soap should we use??(10 votes)
- what is the use of marangoni effect? is it used in scientific fields?(11 votes)
- Some insects propel themselves with biological surfactants (soap).
The Marangoni effect can also drive flow in microfluidic devices - devices that process tiny samples of fluid. In that case the surface tension is lowered by heating part of the fluid surface rather than adding soap, as surface tension is sensitive to temperature.(6 votes)
- What would happen if I replaced the water with soap, and the soap with water?(6 votes)
- Then the soap would be attracted to the water, and nothing would happen. However if you change the boat design so that it has slits at the front instead of the back, it might work.(2 votes)
- Can I just dip the cotton swab in the hand soap. Will the pepper still run if I use hand soap instead?(1 vote)
- So, what shape works best for the boat?(2 votes)
- I stuck a cotton swab with soap on the tip behind the boat and it worked. But when I tried it again it didn't move. Has anyone else had this problem? And does anyone know how to fix it? Thanks in advance!(2 votes)
- What if you used the same amount of soap and the same sized boat that they used ,but the container you put them in was twice as big as theirs. Would this make the boat move twice as slow?(3 votes)
- No, for the Marangoni effect only depends on the surface energy differential. The surface area of the two distinct areas of differing surface energies does not affect the Marangoni effect (no pun intended). You could do this experiment out on the high seas and it would STILL work.(2 votes)
- Will the type of paper change the way the boat moves?(2 votes)
- If you think about it, a lighter paper (like a thin plastic, as opposed to, like, cardboard) would have less inertia, and therefore more net force coming out.(2 votes)
How do you think this boat is being propelled? There's no motor, and there's no sail. It turns out that when we add sop to the end of this toothpick and place the toothpick in the water, something called the Marangoni Effect pushes the boat. What is the Marangoni Effect? It's caused by a special property called surface tension. Surface tension is a tensile or contractile force on a surface. It acts similar to a stretched elastic membrane, kind of like a balloon. Every portion of the surface is pulling in on itself with a contractile force. If a liquid has a high surface tension, it means that this contractile force is high. What happens when you put a drop of liquid with a low surface tension in a bath of liquid with a high surface tension? Let's use water and soap as an example. Water has a high surface tension, while soap has a low surface tension. For each liquid, there's a contractile force on the surface. But since water has a higher surface tension than soap, the contractile force on the surface is bigger for water than it is for soap. In other words, water surface tension will pull more strongly than soap surface tension. This results in a net force from regions with low surface tension to regions with higher surface tension. This net force causes what is called the Marangoni Effect. The Marangoni Effect says that fluid will want to flow from areas of lower surface tension to areas of higher surface tension. We can see an example of the Marangoni Effect by adding pepper to water. There's soap on the end of this cotton swab. Watch what happens when the cotton swab is put into the water. The pepper flakes move away from the point where we added the soap. We can also see the Marangoni Effect cause this string to expand when we add soap to the middle of it. Fluid is flowing away from the region of low surface tension, that is the region with soap, causing the string to expand. Now, let's look back at our soap-powered boat. The Marangoni Effect is what is causing the boat to move. Let's look at a force diagram. As we learned before, the Marangoni Effect states that fluid will flow from the soap region towards the water region. There's fluid coming out of the back of the boat because of the net force between liquids with different surface tensions. The fluid coming out of the back of the boat is what propels the boat forward, kind of like how rockets eject gas to propel forward. This kind of propulsion works because of Newton's Third Law, which states that every action has an equal and opposite reaction. The action of the fluid coming out of the back of the boat produces a reaction in the opposite direction, propelling the boat forward. Why don't you try making your own soap-propelled boat at home? All you need is scissors, cardboard, toothpicks, water, and soap. The cardboard should be somewhat waterproof so it doesn't soak up water. The sides of a milk carton or juice box should work fine. Cut a boat shape out of the cardboard. Then cut a channel shape into the back of the boat. You can try different channel shapes and see which shapes work better. Next, place the boat carefully in the water so that it's floating on the surface. Put soap on one end of a toothache, and put this end of the toothpick in the channel and watch your boat go. You can also try different liquids and see which other ones can propel your boat. [MUSIC PLAYING]