Enhanced Mobility of Granular Mixtures of Fine and Coarse Particles J.C. Phillips(1), N.H. Thomas(2), A.J. Hogg(3) and R.R. Kerswell(3) (1)Centre for Environmental and Geophysical Flows, Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, U.K. (2)Flow Research Evaluation Diagnostics Ltd, Aston Science Park, Birmingham B7 4BJ, U.K. (3)Centre for Environmental and Geophysical Flows, School of Mathematics, University of Bristol, University Walk, Bristol BS8 1TW, U.K. Granular flows that occur in nature as rockslides and volcanic block-and-ash pyroclastic flows, and those arising from industrial processes such as mining and solids conveying, typically contain a range of particle sizes. Previous granular flow studies using two particle sizes have focussed on segregation effects that occur when the mixture flows or is vibrated, and numerical simulations of rapid granular flow have investigated how stresses vary with relative particle size and concentration. We present laboratory measurements of the propagation of flows of fine and coarse granular materials that show that their interaction can result in significantly increased mobility. At a fine material volume fraction of about 0.3, the flow mobility can be increased by up to a factor of four compared with the same volume of either all fine or all coarse material. We interpret these results in terms of the flow frictional regime being predominantly controlled by sliding of coarse material at low fine material volume fraction, rolling of fine material at intermediate fine material volume fractions, and particle interactions within the fine material at high fine material volume fraction. We present model calculations to support this interpretation.