percolation(滲濾) in coffee brewing

 

 

 In physics, chemistry and materials science, percolation (from Latin percolare, "to filter" or "trickle through") refers to the movement and filtering of fluids through porous materials. It is described by Darcy's law. Broader applications have since been developed that cover connectivity of many systems modeled as lattices or graphs, analogous to connectivity of lattice components in the filtration problem that modulates capacity for percolation. 

Darcy's law

Defines the relationship between the instantaneous flux q  (q = Q/A, unit: (m3 of fluid/s) / m2) through a porous medium, the permeability  k of the medium, the dynamic viscosity of the fluid μ  , and the pressure drop ∇ p  over a given distance L

This equation, for single phase (fluid) flow, is the defining equation for absolute permeability (single phase permeability).

 

Those of you who have worked with Darcy's law may not have encountered it in the form above: it is often shown in a simpler form where the pgh term is ignored, because it is often applied in a context where the pressure applied on the fluid is much more important than the fluid's weight (as is the case with espresso). But for pour over coffee, we are in a context of gravity-driven flow, and therefore this more general form of Darcy's law is useful.

The most dramatic one is permeability; in the context of pour over, it is affected by the following variables:

• The grind size (coarser coffee is more permeable, finer coffee is more resistive);

• The permeability of the coffee filter (affected by its pores and thickness);

• The ridges on the inside of the brewer's wall and filter creping (they allow air to flow upward outside the filter and increase permeability);

• The saturation of the coffee bed (a coffee bed saturated with water increases its permeability, which is probably the most important reason why we bloom).

Reference:

why percolation and immersion coffee taste so different

kymos

wiki 

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天文學家的咖啡物理學

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