The Marangoni Effect: How to make a soap propelled boat!

The Marangoni Effect: How to make a soap propelled boat!


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]

11 thoughts on “The Marangoni Effect: How to make a soap propelled boat!

  1. Hello, but what would happen if you add water to to soap? Does the soap rush towards the water?

    How exactly can that be explained using the net force effect you have at 1:20 

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