Percolation is the ability of a liquid-like substance to get through a solid-like lattice. An interesting question is how the likelihood of a material allowing percolation changes as the average density of the lattice changes from 100% (i.e. solid with no percolation) to 0% (i.e. nothing with total percolation). Read an interesting article that looks at the case of square lattices using R: Percolation Threshold on a Square Lattice