Particle Segregation in Dense Granular Flows

Abstract

Granular materials composed of particles with differing grain sizes, densities, shapes, or surface properties may experience unexpected segregation during flow. This review focuses on kinetic sieving and squeeze expulsion, whose combined effect produces the dominant gravity-driven segregation mechanism in dense sheared flows. Shallow granular avalanches that form at the surface of more complex industrial flows such as heaps, silos, and rotating drums provide ideal conditions for particles to separate, with large particles rising to the surface and small particles percolating down to the base. When this is combined with erosion and deposition, amazing patterns can form in the underlying substrate. Gravity-driven segregation and velocity shear induce differential lateral transport, which may be thought of as a secondary segregation mechanism. This allows larger particles to accumulate at flow fronts, and if they are more frictional than the fine grains, they can feedback on the bulk flow, causing flow fingering, levee formation, and longer runout of geophysical mass flows.

Bedload transport: a walk between randomness and determinism. Part 2. Challenges and prospects

“…More recently, the growing perception that bedload transport is a granular flow has given a new impetus to research on grain sorting (Houssais & Jerolmack, 2016). In the field of granular flows, grain sorting is called particle segregation, and it plays a significant role in the dynamics of those flows, for example, by promoting levee formation, self-channelization, mobility feedback and accumulation of coarse particles at the front (Gray, 2018). A few mechanisms have been identified at the particle scale, such as kinetic sieving and squeeze expulsion (small particles fall into gaps that open up beneath them during shear whereas larger particles are squeezed upwards), but understanding of these mechanisms is still only partial.…”

“…A few mechanisms have been identified at the particle scale, such as kinetic sieving and squeeze expulsion (small particles fall into gaps that open up beneath them during shear whereas larger particles are squeezed upwards), but understanding of these mechanisms is still only partial. At the bulk scale, segregation can be described using nonlinear advectiondiffusion equations (Gray, 2018). Extending segregation theory to bedload is attracting growing interest, but this is far from being a simple exercise.…”

Self-channelisation and levee formation in monodisperse granular flows

“…Field observations (Wilson & Head 1981;Bartelt et al 2012) suggest that the margins of the flow may be differentially fluidised during emplacement of levees, which implies that the rheology is inhomogeneous (Iverson & Vallance 2001;Iverson 2003). In debris flows, the evolving pore fluid pressure (Iverson 1997;Iverson & Denlinger 2001) and particle-size distribution due to segregation (Gray 2018) significantly alter the bulk flow by generating drier bouldery margins that resist the motion and form pronounced levees (Johnson et al 2012). Although these effects present additional modelling challenges, the heterogeneous particle-size distribution of a deposit also potentially provides considerably more information about the flow that created it than the deposit geometry alone.…”

Viewing Earth’s surface as a soft-matter landscape

“…clear connections also exist to other amorphous earth materials such as rock and ice. For many of these topics, there are prior reviews of key soft-matter concepts and techniques that we will lean on: rheology and yielding in soft materials [31][32][33][34][35][36] and their application to geophysical flows [37]; jamming [38] and glassy dynamics [28,32,36]; granular segregation [39]; and a variety of experimental soft-matter [40] and granular [41] techniques.…”