When particles are suspended in a highly viscous oil and allowed to flow down an incline, there are two qualitatively distinct regimes: at low particle concentration / inclination angle, the fluid and particles separate; at high particle concentration / inclination angle, the particles stay mixed in the fluid and become more concentrated at the leading front. In some experiments we ran, we noticed the particles and fluid stayed well mixed and didn't bifurcate over the length of the incline. It seemed odd...

We have used a Suspension Balance Model, a model for shear-induced migration (how shear can overcome gravity), and studied the system on a 'fast timescale'. Our model allowed us to explain why the bifurcation didn't take place: in our experimental regime, we were very close to a 'critical particle concentration', where the lengthscale for the bifurcation becomes very large. Given some fitting parameters, we could also qualitatively match other experimental data of similar systems.

We are currently investigating more exotic systems with multiple species present. Already some unexpected phenomena can emerge. We are working on develping a general theory of these more complicated systems.