Going loopy for bubbles

20 July 2008

Ingredients

A coat hanger A coat hanger or other source of wire Some Wool Wool or string
Washing up Liquid Some washing up liquid A tray A tray

Instructions

The bubble loopFirst bend the wire into something that is roughly a circle.

Wrap the wool or string around the wire to form a loop of wool covered wire.  This will act as a reservoir for the bubble mixture making the film last a lot longer.

Tie two fairly loose pieces of wool across the middle of the wire loop forming 3 sections.

Mix a generous amount of washing up liquid (3-4 times as much as you would use for a whole bowl) with water in your tray.

Place the wire loop in the mixture and gently remove it leaving it covered with a bubble film.

Pop the central area of film, what happens.

Try popping the others, and starting again and popping them in another order.

Result

You should find that the wool is pulled away from the area of film you popped.

Explanation

Water and all liquids have a property called surface tension, this acts as a force trying to make the surface of the liquid smaller.  The washing up liquid you added to the water will have reduced this surface tension enough to form a bubble film, but it is still there. This means that when you first form a bubble film on the loop, the film is in tension pulling on all of its edges.  The loose strands of wool are pulled from both sides, so the forces cancel out and the wool stays loose.

Forces in a bubble film The central film popped
The film's surface tension pulls on both sides of the wool so it doesn't move. When the central film is popped there is only a force on one side of the film so the strands are pulled apart.

Forces in a bubble skinWhen you pop the central film, you're left with a film on only one side of the wool, so it can be pulled sideways by the surface tension, allowing the film to shrink.

This surface tension is the reason that bubbles are spherical (ball shaped) this is the shape with the smallest area for a given volume of air inside, so if there are no edges surface tension will always pull a bubble into a sphere.

What is Surface Tension?

Surface tension is caused because the molecules in a liquid are attracted to one another (otherwise they would wander off and it would no longer be a liquid).  In the centre of the liquid these attractions will act in all directions, and apart from stopping the liquid falling apart they have very little effect.

However at the surface of a liquid there are no liquid molecules above, so a molecule on the surface will feel an overall attraction back into the liquid.  This means that unless something is holding the surface up, it will tend to shrink, this tendency we call surface tension.

Water molecules Water molecules on the surface
In the middle of a liquid a molecule is pulled in all directions so there is no overall force. On the surface there is an overall force back into the liquid, this means that unless the surface is held up it will shrink.

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