The Physics of Dialing In: What a Volcanologist's Model Reveals About Your Morning Shot

Every morning, behind the bar, the same ritual unfolds. A barista grinds a dose, tamps it level, locks in the portafilter, and watches the stream. Too fast, too thin, too blonde. Grind finer. Try again. The shot pulls slower now, darker, but the taste skews bitter. Back off half a notch. This dance between intuition and adjustment is the daily reality of espresso, a process that feels equal parts science and guesswork.

A study published in April 2026 in Royal Society Open Science offers a framework for understanding why that guesswork feels so unpredictable. Led by Fabian Wadsworth from the Department of Earth and Environmental Sciences at Ludwig-Maximilians-Universität München, the research applies principles from geology to the coffee puck, modelling how water flows through packed beds of ground coffee.

Why Sofia's Baristas Should Care

In Sofia, where the specialty coffee scene has grown from a handful of pioneers to over sixty cafes in the past decade, many baristas learn through trial and error. The city's coffee culture is increasingly professional, with roasteries like Blue Bag Specialty Coffee working directly with verified farms to source beans at peak ripeness. Understanding the physics of extraction is the natural complement to that level of sourcing precision. This research offers a shared language for what skilled hands already sense: small changes at the grinder produce outsized effects in the cup.

The Core Problem: Permeability

Permeability determines how easily water moves through the coffee puck. If the puck is too permeable, water rushes through, under-extracting the coffee and producing a thin, sour shot. If the puck is too resistant, water struggles to pass, over-extracting and skewing bitter. Baristas control permeability by adjusting grind size, dose, and tamp pressure, but the relationships between these variables are not straightforward.

How the Researchers Tested It

The team used X-ray computed micro-tomography to scan packed beds of ground coffee, then ran fluid-flow simulations through the resulting 3D structures. They tested two coffees roasted by UK-based Square Mile Coffee Roasters: Tumba from Rwanda and Guayacán from Colombia. Each was ground at eleven settings on a Mahlkönig grinder, from very fine to coarse.

Two theoretical models were compared. The first, the Kozeny-Carman equation, is a classic approach used in soil science and engineering. The second, a percolation-theory model, accounts for the possibility that pore networks stop conducting flow effectively once packing becomes too tight. The percolation model matched the coffee data better, especially in denser, lower-porosity samples.

What This Means Behind the Bar

The study confirms what many baristas notice intuitively: grind size and dose are the dominant factors affecting permeability, while tamping plays an important but secondary role. Finer grinding slows water flow, but not by a fixed amount from one grind step to the next. The relationships among particle size, specific surface area, and permeability are nonlinear. A small adjustment at the grinder can produce a disproportionately large shift in resistance and shot time.

This explains why dialing in can feel so unpredictable. Moving one notch finer might barely change the shot, while the next notch transforms it entirely. The math is not linear, and neither is the experience.

Grinder calibration differs by burr set and manufacturer. Real-world complications remain: fines migrate during extraction, and coffee particles swell as they absorb water. These factors are not yet fully captured by any model.

Part of a Growing Body of Work

This research joins a growing body of espresso science. A 2023 paper in Physics of Fluids helped explain why finer grinds can sometimes produce weaker espresso by promoting uneven extraction. Christopher Hendon, a co-author of the new study, also co-authored a 2020 paper arguing that lower doses and coarser grinds could produce more even and reproducible espresso, challenging some traditional recipe assumptions.

Science is catching up to craft intuition. Baristas are not guessing. They are solving fluid dynamics problems with their hands, adjusting variables in a system that resists simple rules. The research does not replace that skill; it illuminates why the skill matters.