Bidisperse kinematic sedimentation

A particle suspension contains two particle species with separate particle volume fractions. In an independent-settling model, each species obeys its own flux conservation equation, while a shared stationary deposit can couple the jump conditions through its maximum total packing fraction. Such independence is an imposed model; general hindered settling need not decouple species.

Initially, separate clearing fronts of fast and slow species coexist with a mixed deposition front. After the fast clearing front meets that deposit, the remaining suspension contains only the slow species. Its later sedimentation shock builds a pure-slow layer above the earlier mixed deposit. Final layer thicknesses and composition follow from separate particle-volume conservation.

For a bidisperse kinematic sedimentation model with normalized suspension concentrations $q_i$, isolated speeds $W_i$, and a stationary deposit with $p_1+p_2=1$, the upward deposit-front speed magnitude obeys $s(p_i-q_i)=W_i{q}_i(1-q_i)$. Adding gives $s={\sum }_i{W}_i{q}_i(1-q_i)/(1-{\sum }_i{q}_i)$; substitution determines each deposited fraction. These are Rankine-Hugoniot conditions with a stationary compacted branch.