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The algorithm for nonwetting fluid injection has been previously described , and is a geometric method that works only for completely nonwetting fluid injection, in 2D, with a contact angle of 180 degrees. The algorithm begins by surrounding a porous image with a bath of fluid pixels. A pressure is implicitly chosen , by selecting a diameter that is the smallest channel through which the fluid will be allowed to go. The fluid is then sucessively intruded from the outside by trying to place fluid circles of the chosen diameter, centered at previously intruded fluid pixels.The circular intruding shape gives apaproximately the correct meniscus, and the chosen diameter guarantees that the fluid will only go into allowed regions. By keeping track of how much pore area was intruded with each choice of the pore diameter, an approximate pore-size distribution can be traced out. Figure 2 shows complete pore-size distributions, averaged over ten realizations of a 60% and a 35% porosity 1000x1000 system. Each curve has been normalized by the total porosity. The value of dc is determined directly by using a "burning algorithm"  to determine when the intruded fluid has percolated. The value of dc is then the intruding fluid circle diameter at percolation, avergaed between the left-right and up-down thresholds, which are not always the same due to finite-size effects .
Figure 2: Two pore-size distributions, computed using the nonwetting fluid injection algorithm, and averaged over ten configurations of a 35% and a 60% porosity system of circles deposited in a 1000x1000 unit cell. The curves are normalized by the porosity.