On the lifetime of a coffee drop

Area of Science:

• Physics
• Fluid Dynamics
• Materials Science

Background:

• Coffee drops form circular stains after evaporation, a phenomenon studied for decades.
• The role of contact angle hysteresis in pinning the liquid during evaporation has been underestimated.
• This pinning effect explains the large stain size observed after coffee drop evaporation.

Purpose of the Study:

• To investigate the consequence of strong contact line pinning on evaporation kinetics.
• To determine if pinned liquid systems evaporate faster than unpinned systems.
• To explore the generality of this effect in various complex liquids.

Main Methods:

• Studying the evaporation of coffee drops and comparing it to water.
• Analyzing the effect of substrate modification (etching a groove) on evaporation rate.
• Observing the evaporation of diverse complex liquids.

Main Results:

• Contact angle hysteresis is significantly amplified in coffee drop evaporation, leading to liquid pinning.
• The pinning effect accelerates evaporation kinetics by approximately 30% compared to water.
• Etching a groove in the substrate, which blocks the contact line, further accelerates evaporation.
• The phenomenon's generality is explored across different complex liquids.

Conclusions:

• Strong contact line pinning, caused by amplified contact angle hysteresis, accelerates evaporation.
• Systems that impede contact line movement enhance evaporation rates.
• The findings have implications for understanding evaporation dynamics in various complex fluid systems.