A recent paper by Yang et al. [1] shows interesting results on the percolation threshold, the ratio of real contact area to nominal contact area A/A0 when the first leak channel appears across a seal interface. Percolation is a very general phenomenon, which perhaps originated from the problem of conduction in mixtures where Bruggeman gave his “effective medium theory” or “effective medium approximation” (EMA) [2] already in 1935. It is known that the threshold values are in general not correctly predicted in three-dimensions [3] (factor 2 errors observed in experiments), but in two-dimensions, the EMA turns out to model percolation relatively well, with a prediction of a threshold of 50%. In the context of the flow in a seal between rough (anisotropic) interfaces; today, it can be said that EMA gives the percolation threshold, the ratio...

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