MadSci Network: Edible/Inedible Experiments Archive

Surface tension driven boat.

Area of Science: Physics
Meant for Grade K-3 (age 5-7).
This experiment is inedible.
An adult need not be present.

Shows how a macroscopic object (foil boat) may be driven across a surface of water by an expanding front of soap molecules.

Piece of foil, scissors, large (preferrably flat) dish or cooking pan filled with water, drop of any dishwashing/laundry detergent or soap solution.

Don't be silly, don't drink soap. If you do, you know what happens. ; |-

How to do the experiment:
Fill a large cooking pan with water, in order to create a reasonably large water surface. Get a piece of cooking foil, and smooth it by "ironing" it with a fingernail. Press the foil against some flat surface to ensure that the surface is flat. Cut a small "boat" out of foil (general shape shown below, dimensions should be reasonably alike the actual picture on the screen, but the precision is not important)

 !       \
  =>      >
 !       /
The back of the boat has a channel cut through it with a small cavity at the end of the channel, something like this (enlarged)
 !      ----\
 ------/    !
 ------\    !
 !      ----/
Smooth the boat gently, in oder to eliminate all the irregularities created by cutting.

Place the boat (!)gently(!) on the surface of water, nose pointing towards a reasonably large area of free water surface. If everything is done with care, it will hang, suspended by surface tension forces. Dip a match in detergent or soap solution, and gently deposit a (!!)small(!!) drop of the solution on the boat, so that it touches the cavity. The boat should accelerate rapidly, like a "rocket" and skid a reasonable distance before stopping.

When you place soap solution in contact with water, soap molecules try to spread over the surface of the water, at first, since they are confined in the cavity of the boat with only one way out they jet from the rear end of the boat creating a reaction force strong enough to drive the boat across the water. As soon as all the water surface is covered with a monolayer of soap molecules, the motion stops. To facilitate the experiment, one may place a small crystal of camphora in the cavity, instead of soap. The boat motion would last much longer.

Useful References:

Further comments:

Experiment submitted on Tue Mar 4 15:54:47 1997 by:
Name: Artem G. Evdokimov
Institution: WIS
Position: phd student

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