Maciej Lisicki and his team at the University of Warsaw have spent months doing what many of us dream of doing: turning coffee into research. The theoretical physicists brewed hundreds of cups of espresso, not for the caffeine, but to answer a question that baristas at the Warsaw Coffee Conference had been puzzling over for years: why does water sometimes channel through a coffee puck unevenly, leaving some grounds under-extracted and others over-extracted, producing a bitter, inconsistent brew?
The answer, it turns out, lies in a material property that scientists call poroelasticity. At lower brewing pressures, coffee grounds behave like any ordinary porous medium — squeeze water through sand, and the flow rate responds predictably to changes in pressure. But espresso, typically brewed at six to nine times atmospheric pressure, lives in a different physical regime. After about 30 to 40 seconds of brewing, the coffee fully dissolves, leaving behind what Lisicki's team classifies as a poroelastic material — one that stops responding predictably to pressure. Increase the pressure, and the flow rate stops climbing accordingly. Something unusual kicks in.
"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."
The team developed a coarse-grained mathematical model to describe this variability, one that also sheds light on the dissolution dynamics happening in those crucial early seconds of extraction. It's the kind of insight that could eventually help baristas dial in more consistent shots, but Lisicki seems equally excited about the pure curiosity of it all.
"As physicists, we turn coffee into research on a daily basis," he said. "Now we made it the subject, and not the fuel."
The research, published in Physics of Fluids, is just the beginning. The team plans to use glass beads in future experiments to visually track water movement through the puck — a brewing process usually hidden inside the machine. It's slow, painstaking work, the kind that builds one small brick of understanding at a time. But for Lisicki, that's exactly the point.
"Physics is about answering mysteries that are unsolved," he said, "and it turned out that coffee has these mysteries in it just as well as galaxies do."