The next time you use a straw to blow bubbles into your juice or use a bubble wand to make soap bubbles, take a moment to observe how the bubbles form and how long they last. You might be surprised by what you see.
Observing the differences between bubbles
If you use a straw to blow air into a glass of water, you can make bubbles. The bubbles tend to be small, and they disappear quickly. Blowing bubbles in juice or a soft drink gets the same result – lots of little bubbles that don’t last long.
What happens if you blow through a straw into a cup of soapy water? The bubbles last much longer. Often, there are so many bubbles that they overflow the cup.
Another way to blow bubbles is with a bubble wand. These bubbles have a different shape to bubbles blown into liquids. Have you noticed that this type of bubble, whether big or small, is always round? Even huge bubbles formed by enormous bubble wands ‘wiggle’ until they form a sphere – unless they pop first.
Water’s strange property – surface tension
Imagine a glass of water is sitting on your desk. Unless someone bumps it, the water looks perfectly still. But if you could see the individual water molecules you would see something very different. Water molecules are in constant motion. The molecules touch each other, but they easily roll and slide past one another. The molecules are not rigidly stuck together as in a solid, but there is a very weak force that holds individual liquid molecules together.
The molecules at the surface of water don’t have water molecules above them to cling to, so they develop a stronger bond with the water molecules next to them. As a result, the surface of the water develops a thin, flexible ‘skin’. Scientists call this surface tension.
Surface tension behaves a bit like an elastic layer. Some insects can walk on water because their weight is small enough so it does not break through the surface tension. If you look carefully at the insect’s feet, you can see that the water looks as if it is stretched.
Surface tension and bubbles
Surface tension is why air bubbles blown into a glass of water are small and don’t last very long. The water molecules surrounding the air bubbles attract each other, joining together.
Adding soap to the water decreases the force of the surface tension. The surface of the soapy water is more flexible than ordinary water. When you blow air into soapy water, the air stretches the surface. The surface tension in this stretchy layer captures the air and pulls the soapy water into the shape of a sphere. This is because a sphere has the smallest possible surface area for the volume of air it contains. The result is a soap bubble.
Bubbles made from soap and water dry out and pop as the water evaporates. This is one reason why bubbles pop more quickly on hot sunny days or when it is windy. Adding glycerine or sugar to the bubble mixture makes the soap layer thicker. This slows down evaporation and makes the bubble last longer.
Surface tension, soap and dirty things
Have you noticed how much easier it is to clean dirty, greasy hands or plates once you add soap to the water? By reducing the surface tension of water, soaps and detergents allow it to mix with oil and grease more easily. Warm water melts the oils and fats so that it is easier for the soap to work – that is why warm, soapy water is so effective.
Nature of science
Scientists need scientific vocabulary to communicate effectively. The term ‘surface tension’ is the correct term to use, but people sometimes use phrases like ‘stretchy layer’ or ‘skin’. Even though these terms are not technically correct, they help people visualise a complex concept.