Froth on a coffee...

[fee-is-me]

Never been in Science before but feel i need to venture in as i am curious about something.

You know these 'Nescafe Cappuccino To Go' things you can buy from supermarkets? (They have coffee/milk powder in the base of a disposable cup that you simply pour hot water onto and stir.)

Well, i fill the cup to where it says to and give it a good stir and then 10 seconds later the level has dropped drastically. There is still loads of froth on the top, so it's not just the froth disappearing before i have drunk any.

I top it up, stir it, and then 10 seconds later it has dropped again.

Does anyone know why?

[ugly_bob]

some of the froth has gone.
When you stir the drink, you put air into the milk (like whipping cream or beating an egg) causing the froth. The froth is not 'strong' enough to contain all the air, so some escapes and the froth shrinks.

[fee-is-me]

Oh that makes sense!

Thankyou :0)

[97026]

The froth is created by a chemical reaction, usually a carbonate. The froth is sustaned by somthing called a surfactant and is separated from the main body of liquid by surface energy. but stirring you are introducing more energy into the system and over coming surface tension, then by dissolution the "froth" is dissolved into the main body. The main part of this being water, a solvent.

[fee-is-me]

Oh.

I shall never stir my coffee without thinking about surface energy again!

Thankyou.

[heathfield]

The fats in milk help reduce the rate of evaporation of the moisture from which each bubble is formed. (97026's 'surfacant'). The instant cappuchino lacks these fats, so the moisture is able to evaporate more quickly, and the bubbles collapse sooner.