Regarding static granular mixtures (pile of sand - particles with different sizes), is there an expression that will give me the entropy of such a mixture? I am interested in finding the entropy of the system initially and once it has been screened ie once the larger particles get sorted out, giving two piles (I think that ths is a reversible process, correct me if I am wrong). Would this mixing entropy be consistent with a bulk entropy such as $\Delta S = m c_p ln(T)$ ? Could I use this bulk entropy and the mixing entropy together in some free energy expression? I have looked for some references but I get nothing that I can access.