For a compressible filter cake, where along the cake thickness is porosity usually the minimum (i.e., most compacted)?

Introduction / Context:
Compressible cakes compact under increasing effective stress. Since pressure drop accumulates through the cake, compaction varies with depth, affecting permeability and specific resistance.

Concept / Approach:
The local pressure is highest near the filter medium because the entire upstream pressure drop is transmitted through the underlying cake. Thus particles are packed tighter there, giving smaller voids (lower porosity) and higher local specific resistance. Upstream layers experience lower compressive stress and therefore exhibit higher porosity.

Final Answer:
At the filter medium (downstream face)