In yesterday’s X-Peri-Mas we used surface tension to move coloured liquids across a liquid surface. We broke the surface tension of water using soap and the colours flew away from the soap. I tried to explain at the end about how the force on the colours is more of a pull from the water than a push from the soap.

This effect is known as the Marangoni effect. Do any of you dream of having an effect named after you?

The water surface is like a balloon – the skin is being stretched and pulled in all directions. This ‘pull’ force across the whole surface creates tension and that makes it hard for things to get through the surface. This surface tension creates a skin on the water that is really useful for bugs that walk on the water surface or for the experiments we’ve seen already in X-Peri-Mas.

A good way to see this, and to use it for something useful, is to make the surface tension pull a boat across a water surface. To do this, we need to add soap to the back of the boat so that the pull force in front of the boat is stronger than the pull force behind. To start with, the boat is surrounded by water – being pulled equally in every direction.

Being pulled in every direction at once means the boat goes nowhere!

Some molecules don’t pull on their neighbours as strongly as water molecules do, though. Soap definitely doesn’t – soap molecules are a bit lazy and weak and don’t pull on the molecules around them at all strongly. This means that far away from the soap – where there is still water at the surface – there is still a strong pull force but where you’ve put the soap the pull force is not as strong! Think of having a tug-of-war where one side is not pulling as strongly as the other: the weaker side will be pulled towards the stronger side.

In our liquid the soap, not pulling as strongly, is pulled across the surface by the liquid further away that is still mostly water so pulls more strongly.

The soap dripped into the back of the boat weakens the pull behind the boat so the pull at the front is strongest and… whoosh! Off the boat goes.

What you will need

  • A piece of card
  • A tray of water
  • Soap
  • Something thin and long like a toothpick or piece of spaghetti

How to do it

You’ll need to cut out a small boat shape and then add a channel into the back. This is where you are going to add the soap. Here’s an example I made from wood but I’ve also made these with plastic and card too!

Add a tiny drop of soap to the channel in the back of your boat using your thin, pointy stick.

Get involved – make one at home and share your creations on twitter with #XPeriMas!

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