Make a miraculous Marangoni soap boat
Victorian Toys and
Flatland Rockets
Written and illustrated by Bob Knetzger
BOB KNETZGER is a designer/
inventor/musician whose award-
winning toys have been featured on
The Tonight Show, Nightline, and
Good Morning America. He is the
author of Make: Fun!, available at
makershed.com and fine bookstores.
B
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
).
NOW TRY THIS NEW, MORE DURABLE
VERSION:
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. Visit makezine.com/go/
marangoni-boat to see a video demo.
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
).
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.
A
PROJECTS: Toy Inventor's Notebook
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C
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