Milk Bubbles
by Robert Krampf

This week’s experiment explores a popular childhood activity, blowing bubbles in your milk. It is a great way to learn about surface tension, milk chemistry, and have fun doing things that your Mother told you not to.

To try this, you will need:

• a glass of milk that is half full (or half empty, depending on your personal outlook)
• a glass of water that is half full
• a soda straw

The start is easy. Put the straw in the glass of milk. If the glass is more than half full, use the straw to drink the excess. Then blow gently through the straw, making bubbles in the milk. Continue doing this until you fill the glass with bubbles, or until your mother tells you to stop playing with your food and get ready for school. (Oops. Sorry. Childhood flashback.)

It was pretty easy to fill the glass with milk bubbles. Next, try the same thing with the glass of water. This is not nearly as easy. The bubbles pop very quickly, making it difficult to fill the glass with bubbles without blowing so hard that you make a mess.

Why the difference? Water molecules are very sticky, attracting each other very strongly. At the boundary between water and air, they make up for the lack of attraction above the surface by pulling even stronger to the sides. This forms a skin-like surface tension on the water. That surface tension is what pulls water into beads on a freshly washed car.

A bubble is made up of a thin film of water. With pure water, the pull of the surface tension make the water film so thin that it pops almost instantly. For the bubble to last longer, you need some way to reduce the surface tension.

Milk contains proteins. These proteins are long, string-like molecules that form a network in the bubble reducing its surface tension. Less surface tension lets the bubbles last longer, making it easier to fill the glass.

The amount of milk fat can also have a big impact on this. Liquid milk fat forms films in the bubble more easily than the milk protein, but since the fat does not mix with water, it does not reduce the surface tension. That makes weaker bubbles, so low fat milk tends to make better bubbles than whole milk.

Temperature also has an impact. With a glass of cold milk, the bubbles were large and lasted quite a while. As the milk warmed up to room temperature, the bubbles were smaller and popped quickly. This means that you should blow your milk bubbles early in the meal, instead of waiting to have bubbles with your desert

Reprinted with permission. © 2008. Robert Krampf’s Science Education