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Abstract

A recurrent problem in materials science is the prediction of the percolation threshold of suspensions and composites containing complex-shaped constituents. We consider an idealized material built up from freely overlapping objects randomly placed in a matrix, and numerically compute the geometrical percolation threshold, p_c, where the objects first form a continuous phase. Ellipsoids of revolution, ranging from the extreme oblate limit of plate-like particles to the extreme prolate limit of needle-like particles, are used to study the influence of object shape on the value of p_c. The reciprocal threshold, 1/p_c (p_c = critical volume fraction occupied by the overlapping ellipsoids), is found to scale linearly with the ratio of the larger ellipsoid dimension to the smaller dimension in both the needle and plate limits. Ratios of the estimates of p_c are taken with other important functionals of object shape (surface area, mean radius of curvature, radius of gyration, electrostatic capacity, excluded volume, and intrinsic conductivity) in an attempt to obtain a universal description of p_c. Unfortunately, none of the possibilities considered proves to be invariant over the entire shape range, so that p_c appears to be a rather unique functional of object shape. It is conjectured, based on the numerical evidence, that 1/p_c is minimal for a sphere, of all objects having a finite volume.

• Introduction
• Basic functionals of object shape and their evaluation for ellipsoids of revolution
  • Surface area
  • Mean radius of curvature
  • Radius of gyration
  • Electrostatic capacity
  • Excluded volume
  • Intrinsic conductivity
  • Asymptotic limits of shape functionals
• Estimation of percolation thresholds using shape functionals
• Percolation algorithm
• Results and Discussion
• References