The instability criterion for bidisperse sedimentation derived by Batchelor
and Janse van Rensburg was combined with two empirical models for. the Par
ticle velocities that are consistent with conservation of momentum and wed
to construct theoretical stability diagrams. The stability boundaries were
fully determined by the particle volume fractions, the particle-size ratio,
and the ratio of the density differences with the liquid. The results show
ed that on increasing the particle-size ratio sufficiently stability always
is attained. For particles of similar size, instabilities occur mainly if
the liquid density is intermediate between the particle densities. The stab
ility diagrams agree well with the limited experimental data, if so-called
"vertical columns" are excluded This seems reasonable since similar structu
res observed in monodisperse suspensions are not captured by the bidisperse
instability criterion either.