Transient dynamics of bidisperse suspensions on an incline

We experimentally investigate the influence of particle size and volume fraction on bidisperse suspensions of negatively buoyant particles released down an incline. Side profile analyses of the thin film at equilibrium reveal the Brazil nut effect, where large particles accumulate at the free surface and small particles concentrate near the bottom. At relatively low volume fractions and inclination angles, corresponding to the settled regime, we observe transient behaviors before equilibration. Here, large particles initially settle to the bottom, causing the size-based segregation pattern to flip—the reverse Brazil nut effect. In bulk flow, this transient manifests as small particles initially forming a leading front; only later do large particles catch up and overtake the front. We observe particle diameter significantly impacts transient behaviors, with smaller diameters resulting in longer transient times. Meanwhile, decreasing volume fractions also leads to transient behaviors. We also report further work on how size ratio between particle species affects transient dynamics, comparing our findings with literature on dry granular flows. In those studies, the Brazil nut effect occurs at moderate size ratios; but when the size ratio exceeds a threshold the reverse Brazil nut effect emerges. We highlight important distinctions between wet and dry granular flows, providing new insights into the role of particle and fluid interactions in dynamics.