Science has an interesting answer! As a starting point for the week, let’s explore a Hamlet-like dilemma that many people face every day, often without thinking about it: what is the best method for mixing milk and coffee, while keeping the temperature as high as possible? 1) Pour the coffee into the milk already in the cup, 2) Pour the milk into the coffee. We simulated both scenarios and the results show that solution 1 allows us to maintain a higher average temperature.
But why does this happen? When we pour colder and therefore denser milk into hotter coffee, the higher density of the milk causes it to tend to settle quickly on the bottom of the cup. This limits the mixing between the two liquids and reduces the heat exchange area, leaving two zones at different temperatures. On the contrary, pouring the coffee into the milk triggers more effective mixing right from the start.
In fact, the falling coffee generates kinetic energy that pushes the milk towards the surface, while the milk, having a higher density, will tend to return to the bottom. We can consider these movements as forced convective currents that therefore increase the interaction between the two substances, improving the distribution of heat. The result? Solution 1 makes the most of the principles of physics to keep the drink hot for longer. So, if you want to enjoy your latte macchiato at the ideal temperature, next time pour the coffee into the milk and let science do the rest!