Why Does Coffee Leave a Stain Around the Cup Rim?

Discover why coffee leaves a stain around the cup rim and how to prevent it.

Why Does Coffee Leave a Stain Around the Cup Rim?
Photo by Olena Bohovyk / Unsplash

When you take a sip of coffee, have you noticed a lingering stain around the top surface of the cup, while the rest of the liquid remains clear?

This commonplace phenomenon can be explored through the lens of science and physics, shedding light on the complexities of fluid dynamics and evaporation.

The Role of Evaporation in Coffee Stains

As the top layer of the coffee is exposed to air, evaporation comes into play.

The water in the coffee evaporates, leaving the solids (coffee particles and oils) behind.

This process is accelerated at the liquid's surface due to the larger surface area.

As the water evaporates, the solids become more concentrated, leading to the formation of a visible residue - the notorious coffee stain.

Interesting, isn't it?

Why does coffee create a stain and not just evaporate cleanly?

The answer lies in the intricate dance between liquid and air, as well as the behavior of particles at the liquid's surface.