Cappuccino with steamer
Foam has some typical properties of a solid. That's why it acts almost like a lid on the surface of a liquid - and can even prevent it from spilling over with strong movements.
Coffee to go has been popular for many years, but there are often problems transporting the full cup. The rhythm of walking with a drink held by a rigid arm stimulates ripples on its surface. Then it spills over after just a few steps. The phenomenon affects normal coffee with and without milk, but cappuccino is largely spared such losses. The difference between the drinks lies in the way the milk is added: in one case it is liquid and in the other it is foamed - as a kind of topping. The latter seems to have dampening properties. The reason for this lies in the structure of foams.
In everyday life we mainly gained experience with soap foam. Every child knows how this turns into a veritable tower: You blow through a straw into a glass with soapy water until a cluster of numerous bubbles finally squirts out at the top.
Bubbles are, so to speak, inflated with a gas, ideally spherical liquid lamellae. Both inside and outside, they are in contact with the air over a very large area. It takes energy to create this interface…