Your dripper is a physics experiment in disguise

Each morning, countless people engage in a quiet ritual: boil the water, grind the beans, and slowly pour into a dripper. It’s simple. It’s deliberate. For many, it’s a moment of calm, even meditation. But there's a lot of physics behind this everyday act. Water becomes a jet, coffee becomes a granular medium, and brewing becomes an experiment governed by the same equations that describe oceans and rocket engines.

While most brewing guides focus on ratios, grind sizes, or flavour notes, few discuss how water moves through the coffee bed and the importance of this motion. Until recently, the dynamics of pouring were guided by intuition and experience rather than scientific understanding.

That’s beginning to change. In a recent study, physicists used fluid dynamics to model one of the most common yet least understood parts of brewing: the pour. Their findings help us to visualise what is going on inside your dripper and open the door to brewing better coffee by providing a deeper understanding of flow, mixing, and granular behaviour.

Why do we need a scientific framework for pour-over coffee?

The pour-over method is often taught as a meditative, intuitive process. You bloom, you pour, and with enough patience, good coffee flows. But despite its popularity and obsessive optimization by coffee nerds, the mechanics of what’s happening, how water mixes with coffee grounds, extracts flavor, and interacts with particles,  has largely been guesswork.

That's why the study Pour-over coffee: Mixing by a water jet hitting a bed of small rocks with avalanche dynamics (Park et al., 2025)  got people talking not just in labs and research groups but also among people who love coffee. It brings the physics of fluids and granular materials to the kitchen (or wherever you make your coffee).

The result? A deeper understanding of how you pour your water dramatically changes what ends up in your cup.

The jet, the granules, and the boundary layer: what’s going on in your dripper?

At the heart of this research is the interaction between a liquid jet (water) and a granular bed (coffee grounds). This has been studied in other contexts (think of rainfall hitting sand), but never with the level of rigor or application that coffee brewing demands. The authors created simulations using transparent silica granules to replicate coffee, allowing them to visualize interactions that are normally hidden inside coffee that is dark and hard to look through.

The experimental setup featured a glass funnel acting as the coffee cone, captured from the side by a high-speed camera with a macro lens. A thin laser sheet illuminated the flow, aimed perpendicular to both the funnel and the camera, allowing the researchers to visualize the movement of water and particles in high detail.

What they found:

1. Water jet mixing zones:
   
   

• When water hits the coffee bed, it creates a zone of intense movement and mixing near the impact point. This localized region is critical for extraction.
  
  

• Higher pour height (up to 50 cm) increases turbulence at impact and deepens the mixing zone. But beyond a point, the jet breaks into droplets, bad for consistency.
  

2. Pour height vs. extraction:
   
   

• Pouring from a moderate height (20–30 cm) maximizes both mixing and laminar flow beneath the surface. This allows fresh water to access more grounds via the boundary layer.
  
  

• Interestingly, a thicker, faster stream from a high height extracted more than a thin one, despite equal pouring height, due to higher penetration and local turbulence.
  
  

3. Bloom and co₂ degassing:
   
   

• Coffee traps CO₂, which is acidic and hinders extraction.
  
  

• Degassing allows more efficient intra-particle diffusion (i.e., water accessing flavor compounds inside the grounds). Skipping it leads to lower extraction and perceived acidity imbalance.
  
  

4. Avalanche dynamics:
   
   

• As the water hits the bed, coffee particles shift and collapse these “micro-avalanches” expose new surfaces and refresh the boundary layer.
  
  

• A stationary bed leads to under-extraction in deeper layers. So agitation is good (up to a point).
  
  

5. Turbulence isn’t everywhere:
   
   

• Although the jet may be turbulent at impact, flow within the granular bed is largely laminar.
  
  

• This means that extraction is limited by how quickly fresh water can diffuse to coffee particles, not just by turbulence or brute force pouring.
  
  

Granular beds: more than just coffee

The above phenomena aren’t unique to coffee; they reflect a broader set of physical behaviors observed in granular materials, which exist somewhere between fluids and solids. According to (Eggers & Villermaux, 2008), granular beds can rapidly transition between rigid, stable structures and dynamic, flow-like systems when disturbed. This perfectly describes the behavior of coffee grounds under a pour: solid and structured until the moment a water jet triggers collapse, mixing, and flow.

Hydrodynamics and extraction uniformity

The interaction between water flow, bed compaction, and porosity is also explored in Wang & Lim, 2023, who show that uneven flow and compaction, often the result of poor pouring technique, can cause channelling and leave large sections of the coffee under-extracted. The permeability of the bed is dynamic: it changes as the coffee swells, collapses, and saturates.

This directly supports the findings from Park et al, particularly regarding the role of agitation, pour distribution, and stream thickness. In other words, bad pouring creates clogged highways and desert zones inside your bed.

Here’s a data-driven cheat sheet you can use:

What does this mean for the coffee world?

This study signals a new chapter in coffee science: one where physics, not just sensory tradition, guides our brewing. It empowers brewers to tweak one of the least understood variables in manual brewing, the pour itself. And it shows that we don’t have to guess.

Coffee may be an art. But it’s also physics in motion. Every time you pour water into your dripper of choice, you're creating a real-time interaction of liquid jets, granular movement, and diffusion, a miniature experiment at breakfast, noon, or evening, if you are like us at the CR lab😅, we drink coffee all day.

If you're more of a visual learner, take a moment to explore the figures from the original study on ResearchGate. They offer a striking window into the dynamics hidden beneath your coffee bed, from jet impact zones to granular avalanches and erosion patterns. These visuals do an excellent job of translating complex physics into something intuitive and even beautiful.  Seeing how water moves, mixes, and extracts through each layer makes the science click in a whole new way. Highly recommended.

References

• Eggers, J., & Villermaux, E. (2008). Physics of liquid jets. Reports on Progress in Physics, 71(3), 036601. https://doi.org/10.1088/0034-4885/71/3/036601

• Park, E., Young, M., & Mathijssen, A. J. T. M. (2025). Pour-over coffee: Mixing by a water jet impinging on a granular bed with avalanche dynamics. Physics of Fluids, 37(4). https://doi.org/10.1063/5.0257924

• Wang, X., & Lim, L.-T. (2023). Effects of grind size, temperature, and brewing ratio on immersion cold brewed and French press hot brewed coffees. Applied Food Research, 3(2), 100334. https://doi.org/10.1016/j.afres.2023.100334