Miraculous Marangoni Soap Boat
15-30 min
Ages 8+
What Will You Make?
Make a classic Victorian toy that moves thanks to surface tension. We’ll start with a traditional paper version and then craft our own plastic version.
What Will You Learn?
In this activity, we’ll explore surface tension, the Marangoni effect, and forces and motion.
Create Your Toys
Paper Fish
Here’s an update of a Victorian plaything. Cut out the fish on the dotted line and float it on a pan of water. Place a single drop of olive oil in the circle. The oil quickly spreads out the slit and across the water. The fish “swims” in the opposite direction, like an exhaust-spewing rocket subject to Newton’s third law of motion. Sadly, the soggy paper fish is only good for just a single use (Figure A).
Durable Boat
Find a flexible lid from a margarine or yogurt container. Look for the recycling symbol 2 or 4 for low- or high-density polyethylene. (PE is one of the few plastics that floats.) Use a paper punch to make a small circular hole, then cut out the “rocket” shape as shown in Figure B.
Float the rocket in a pan of clean water. Dip the tip of a toothpick in detergent and momentarily touch it inside the rocket’s round hole. As the detergent dissolves, it spreads down the slit and out along the surface of the water — the rocket shoots forward! Touch it again. After a time or two, you’ll have to change the water for the effect to work again.
What Is Happening Here?
Marangoni effect
As the detergent dissolves, it spreads down the slit and out along the surface of the water — the rocket shoots forward.
Another force is also at work: the Marangoni effect, the difference in surface tensions created by the molecules of detergent as they make the water slipperier and “wetter.” The surface tension is reduced behind the rocket, causing the water in front to contract, pulling the rocket forward (Figure C above).
These tensions, forces, and actions all exist at the single-molecule-thick surface of the water — similar to the two-dimensional world in Edwin Abbott’s Victorian-era book, Flatland: A Romance of Many Dimensions.
What' Next?
Toy Inventor's Notebook
Why not try making more classic toys? Try these ideas from Make: Magazine!
This project originally appeared in Make: Magazine in October 2020. The author is Bob Knetzger. Bob is an inventor/designer with 30 years of experience making fun stuff.
Materials:
- Paper
- Pencil
- Scissors
- Paper punch
- Toothpick
- Margarine or yogurt container top
- Pan for water
- Water
- Oil
- Dish detergent
