As the hot espresso is poured into the cold frothed milk, several flow phenomena occur. The temperature difference between the two liquids creates thermal convection, causing the hotter, denser espresso to sink initially, while the cooler, less dense frothed milk rises. This sinking and mixing creates striking vortex patterns. The milk’s frothiness, which consists of trapped air bubbles, adds to the complexity of the flow by introducing buoyancy forces and surface tension effects that further disturb the liquid interface.

As the espresso penetrates the milk, you’ll observe turbulent mixing, creating beautiful, chaotic swirls and streaks, highlighting regions of laminar flow and turbulence. This is an example of Rayleigh-Taylor instability, where the heavier espresso displaces the lighter milk, causing dynamic mixing. The diffusion of heat and mass leads to gradual homogenization, with the espresso visually diffusing into the milk in unpredictable patterns. This can be captured beautifully in slow-motion to showcase the intricate interactions between fluid properties.

music credit

Song: Scott Buckley – Red Roses Realm – Intro
License: Creative Commons (CC BY 3.0)