Milk Science

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Facts About Foam

MS 4.03 Ostwald Ripening

Contact with espresso can destabilise milk foam. Have you ever noticed the border around a nice latte art design start to break up? This is a very slow-developing phenomenon, but a distinct ring of larger bubbles can often be seen around a drink that has been left untouched for five to ten minutes. The white parts of a latte art design that have not come into contact with coffee will be more persistent than the foam which has mixed with coffee and taken on a brown colour. One possible explanation for this breakup of foam can be the gradual merging of bubbles through the diffusion of gas from smaller bubbles into larger ones. This effect is known as Ostwald ripening (OR):

‘Inert gas diffuses through thin films from the smaller to the larger bubbles because of the difference in their capillary pressures,