The difference between milk and water is that milk contains proteins. These proteins are long, string-like molecules that form a network in the bubble reducing its surface tension. So you can blow bubbles in water but they pop very quickly. Reduced surface tension in the milk (caused by the proteins and milk fats) lets the bubbles last longer, making it easier to fill the glass to the top with bubbles. Alex Nicholls
It's yer long chain organic molecules increasing the viscosity, innit. Dr Edwards
It is because milk has lipids... (a fancy word for fatty fat) and proteins that make it wholesome, delicious and bubblectibly "stackable" unlike water. Like making bubbles with washing up liquid, the molecules are more easily arranged into stronger, longer-lasting shapes. Drink milk, don't make a jacuzzi with it. Although it is kinda fun. Brad
An interesting note about frothy coffee!
When you froth the milk for a cappuccino, the cow’s milk captures and suspends air bubbles because of the interaction between proteins and lipids that occurs when they are heated. The process of heating the milk - using a high-pressure steam frother - allows the different molecules to bond with each other, creating a network that holds bubbles in place within the milk. You get better froth with low-fat milk as the fat does not bond with the water as well as the protein. If you use non-dairy milk – such as soya, rice or almond milk, the chemistry that allows cow's milk to create such beautiful, lasting microfoam simply isn't the same so you will get tasty coffee but without the beautiful foamy, bubbly milk! - Lydia
A couple of bubble blowing experiments you can try
The start is easy. Put the straw in a half-full (or half empty!) glass of milk. Then blow gently through the straw, making bubbles in the milk. Continue doing this until you fill the glass with bubbles, or until an adult tells you to stop playing with your food!
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? 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 pudding!
There are six things in milk: water, sugars, fats, proteins, vitamins/minerals and some other stuff like antibodies. One of those things must be causing the bubbles.
Try an experiment. Pour a glass of water and try to blow bubbles in it. There will be bubbles, but not like in milk. Add sugar to the water and see what happens. Still no bubbles. Add a little vegetable oil to the water. It will float on top and not mix in, so this is a little unfair.
Now add some protein. The easiest place to get protein is from an egg, so add a little egg white to a clean glass of water and mix it in. It really does not take much egg white – try half a teaspoon or so. Mix thoroughly. Put your straw in and blow. Bubbles!
So it is the proteins in milk that make the bubbles. It works because the proteins lower the surface tension of water and makes bubbles easier to form.
Note from Blast Science- thanks to Richard for this question - we then did the above experiments in some Eggtooth School Sessions, they worked brilliantly! See below..