The effect of particle shape on suspension viscosity and implications for magmatic flows

Abstract

[1] The rheology of crystal-bearing magma and lava depends on both the shape and volume fraction of the suspended crystals. We present the results of analogue rheometric experiments on monodisperse suspensions of solid particles in a Newtonian liquid, in which particle volume fraction ϕ and aspect ratio rp are varied systematically. We find that the effect of ϕ on viscosity is well captured by the Maron–Pierce model, and that this model is valid across the range of particle aspect ratios investigated (0.04 ≤ rp ≤ 22, i.e., strongly-oblate to strongly-prolate) when the maximum packing fraction ϕm is treated as a fitting parameter. The value of ϕm derived from fitting to our experimental data depends strongly and systematically on particle aspect ratio; hence, ϕm represents an effective proxy for the influence of particle shape on suspension rheology. We present a simple relationship for ϕm (rp) which allows the viscosity of a suspension to be calculated as a function of ϕ and rp only. We investigate the impact of accounting for crystal shape when modelling volcanic flows by simulating the eruption of magma carrying crystals of different aspect ratio, and conclude that the effect of crystal shape should not be neglected.