Enhanced mobility of granular mixtures of fine and coarse particles

Understanding how granular materials flow remains an outstanding issue in both nature and industry where typically a range of particle sizes are involved. We present laboratory measurements of flows of binary mixtures of fine and coarse granular materials that show that their interaction can result in significantly increased mobility (the ratio of the run out distance for the centre of mass to the initial height of the centre of mass). At a fine material mass fraction ψ (the ratio of the mass of fine particles to the total mass of the flow) of about 0.3, the flow mobility can be increased by up to a factor of 4 compared with the same mass of either all fine or all coarse material. Measurements of the distribution of the deposited fine and coarse particles along the length of the flow show that in low mobility flows the peak in deposition of fine and coarse particles occurs at different positions, whereas for high mobility flows there is a more uniform deposition of both particle sizes. We interpret the observations of flow mobility in terms of two primary mechanisms: at low ψ the fine particles lubricate the flow of the coarse particles by rolling, whereas at high ψ the presence of coarse particles reduces the inter-particle frictional losses in the flow. We use heuristic models to illustrate that these mechanisms are likely to occur in granular flows containing a wide range of grain sizes, noting that the detailed physical descriptions and mathematical models of these particle interactions are currently incomplete. These experimental observations challenge the traditional view that the extent of geophysical granular flows depends on their volume, in line with other recent studies of slumping granular materials. Recent studies which use a single particle size show strong dependence of flow run out on the initial aspect ratio of the release. Our results suggest that compositional effects in flows containing more than one particle size may be at least as important.