Physics: The science behind your latte

Pouring hot espresso into warm milk can create distinct layers but only for a specific pouring speed, according to experiments described in Nature Communications this week. The findings help identify a straightforward method to produce layered structures in soft materials.

Distinct patterns can form in liquids when a heat gradient causes a portion of a fluid to heat up and become lighter and another portion to cool down and become denser, giving rise to convection cells. Competition between two different types of gradient can lead to a similar effect called double-diffusive convection, as can be observed in the layering of the ocean due to competing salinity and temperature gradients.

To understand how layering can occur in everyday situations, such as the preparation of a latte, Howard Stone and colleagues perform experiments in an experimental liquid that mimics the coffee-based drink. They identify double-diffusive convection as the mechanism responsible for the appearance of density layers in latte. In this case the experiments reveal that there is a competition between a thermal gradient, due to the difference in temperatures of hot espresso and warm milk, and a density gradient, caused by the injection of espresso. They report a critical pouring speed, which is necessary to produce the layers, whereas at lower pouring speeds the liquids mix but do not separate out into layers. The authors suggest using this example from daily life to easily and repeatedly produce layering in soft materials in an industrial setting.

The proposed layering strategy promises applications in food science and materials science.