The Physics of the Pore: Hydrodynamics, Diffusion, and the Hybrid Extraction of Pure Over

In a standard pour-over, the barista controls the turbulence. The gooseneck kettle creates a stream that agitates the grounds. In the Pure Over Glass Coffee Maker, the machine controls the turbulence—or rather, the lack of it.

This device introduces two structural innovations: a Diffuser Lid and a Rigid Glass Filter. Together, they create a unique hydrodynamic environment that blends the physics of immersion (like a French Press) with the physics of percolation (like a V60). This article deconstructs the flow dynamics of this system, explaining why the “Diffuser” acts as a flow regulator and why the glass pores make “Grind Size” the single most critical variable in your brew.

The Diffuser Lid: Regulating Turbulence

The Pure Over comes with a glass lid that sits atop the dripper. You pour water onto this lid, not the coffee. * The Shower Effect: The lid has holes that distribute the water. This mimics the “Showerhead” of an automatic batch brewer. * Flow Rate Clamping: No matter how fast you pour from your kettle, the lid restricts the entry rate of water into the coffee bed. It acts as a Hydraulic Damper. * Low Agitation: By breaking the single heavy stream into gentle droplets (“raining”), the diffuser minimizes Agitation and Channeling. The water saturates the bed evenly without digging holes in the grounds. This promotes a uniform extraction, essential for a brewing method that lacks the flow restriction of a paper filter.

The Glass Pore: Rigid Filtration Dynamics

The filter is not a separate piece; it is the bottom of the glass dripper itself, perforated with precision holes. * Rigid vs. Flexible: A paper filter is flexible and fibrous. It can expand and contract; fines can get trapped inside the fiber matrix (depth filtration). A glass filter is rigid. The holes are fixed geometry. This is Surface Filtration. * The “Clog” Factor: Because the pores are rigid, they are unforgiving. If a coffee particle is slightly larger than the hole, it bridges it. If it is smaller (a fine), it passes through or plugs it. * Darcy’s Law: The flow rate ($Q$) is determined by the permeability ($k$) of the coffee bed.
$$Q = \frac{-kA}{\mu} \frac{\Delta P}{L}$$
In the Pure Over, the glass pores provide very little resistance compared to paper. Therefore, the resistance must come from the coffee bed itself. This makes Grind Size the throttle. If the grind is too fine, the fines migrate to the glass holes and block them completely (Total Occlusion), stalling the brew. If too coarse, the water rushes through (Zero Retention), resulting in sour water.

Hybrid Extraction: Immersion + Drip

The Pure Over is described as a “Dual Immersion and Dripper.”
1. Phase 1: Immersion (The Stall). When you first pour water (via the diffuser), the grind restricts the flow through the glass holes. The water level rises in the cone. For a moment, the grounds are fully submerged. This facilitates Diffusion—the movement of solids from high concentration (bean) to low concentration (water).
2. Phase 2: Percolation (The Drawdown). As gravity pulls the water down, it flows through the bed. This washes the dissolved solids out. * The Balance: The success of this hybrid method depends on the Drawdown Time. The target is around 5 minutes. This is significantly longer than a V60 (2:30-3:00). The glass filter requires this longer contact time to allow the water to extract flavor fully, compensating for the lack of fine filtration that paper provides.

Conclusion: Engineering the Variable

The Pure Over is a lesson in hydrodynamic constraints. By removing the paper filter, it removes the safety net. It forces the user to engineer the flow rate through grind size alone.
The Diffuser Lid is the assistant engineer, ensuring that the water enters the system gently, preserving the integrity of the coffee bed. Together, they create a brewer that is unforgiving of poor technique (bad grinders) but rewarding of precision, delivering a cup that is texturally complex and chemically pure.