How much time will a dripped drop of coffee be a drop, before it loses its full-bodied nature? This study sniffs into the question:
“On the Lifetime of a Coffee Drop,” Paul Bienvenu, Haruka Hitomi, and David Quéré, Soft Matter, epub 2026. The authors, at PSL Research University, ESPCI-Paris, France, report:
As we know, a coffee drop leaves after evaporation a circular stain whose diameter is generally that of the drop. This effect has been studied in depth for about 25 years, but it has been rarely emphasized that the stain dramatically amplifies the contact angle hysteresis. Accordingly, the liquid can remain pinned from the beginning to the end of the evaporation, which explains the size of the stain. Here we discuss a consequence of this strong pinning. It is found to make the kinetics of evaporation faster than that of water, by about 30%, since a very flat drop evaporates quicker than a bulging one.
Unrelated Bonus: Cough Drop Life Elongation
Though not much related to coffee drops, cough drops, too, have limited lifetimes. This instructive essay touches on that fact:
“Displaying confectionery and cough drops in bulk,” Franklin M. Apple, The Journal of the American Pharmaceutical Association, vol. 4, no. 3, 1915, pp. 309-311.
Apple opines:
Inasmuch as we have a reputation as professional men to protect, it is incumbent upon us to take the initiative as business men to practice cleanly methods in displaying merchandise for sale to the public, thereby demonstrating our knowledge of and respect for hygiene and bacteriology ; and it cannot fail to have a beneficial moral effect. leading the public to look upon us as’ a superior class of merchants to whom they will turn when in need of first-class, dependable goods. I would suggest that we make it an unalterable rule to never sell any commodity to anyone .in a condition that we would hesitate to accept as clean enough for our personal use; and if we make this humanitarian decision, we will not display confectionery or ‘cough drops in a container unprotected from dust, dirt and filth.