WASHINGTON, June 23, 2026 — When a physics student asked baristas at the Warsaw Coffee Conference what their biggest question for scientists was, the baristas said they wanted to know how to stop channeling during brewing.
Channeling is an issue that arises as hot water passes through a pressed "puck" of espresso. The water follows the path of least resistance through the coffee grounds, resulting in an uneven brew and bitter flavor.
In Physics of Fluids, by AIP Publishing, researchers from the University of Warsaw set out to determine the physical properties of espresso brewing to improve the preparation process.
"As physicists, we turn coffee into research on a daily basis," said author Maciej Lisicki. "Now we made it the subject, and not the fuel."
Using a barista-grade coffee machine and a pressure sensor, Lisicki and his team brewed hundreds of cups of espresso at varying pressures.
They found that at lower pressures, the coffee behaves like a regular porous medium, and the amount of pressure of a liquid through a porous medium is directly proportional to its flow and discharge. However, espresso is usually brewed using about six to nine times the pressure of the atmosphere, and Lisicki said that at those levels, something funny starts happening.
After about 30 to 40 seconds, all the coffee dissolves, and what is left can be classified as a poroelastic material. This type of material exhibits nonlinear pressure dynamics, meaning that as the pressure is increased in the espresso machine, the flow rate of the water through the coffee puck no longer increases.
"This poroelastic compaction has been alluded to in the coffee community, but with no systematic evidence," Lisicki said. "I think we characterized this effect for the first time, and that enticed us to formulate a theoretical description."
Lisicki is excited to continue his team's espresso research and hopes to be able to dedicate more resources to understanding the complex physics behind espresso brewing.
"Being theoretical physicists, we were more than happy to jump into the math behind it, and we came up with a coarse-grained model that explains this variability with applied pressure," he said. "It also gives us insights into the dissolution dynamics seen at shorter times."
In addition to studying the motion of the water through the coffee puck, the researchers plan to use glass beads for imaging so they can get eyes on a brewing process usually hidden by machinery.
"Physics is about answering mysteries that are unsolved, and it turned out that coffee has these mysteries in it just as well as galaxies do," Lisicki said.
