Instability in an oscillating non-Brownian iso-dense concentrated suspension

ROHT, YANINA LUCRECIA; IPPOLITO, IRENE; HULIN, JEAN-PIERRE; SALIN, D.; GAUTHIER G.

Puerto Varas

Workshop; Southern Workshop on Granular Materials 2018; 2018

We study experimentally a novel instability marked by the appearance of bands transverse to the oscillating flow of a concentrated suspension of non-Brownian particles in HeleShaw cells. The particles are mono-disperse polystyrene beads of diameter d = 40 µm and are dispersed in a water glycerol mixture of the same density as that the particles (ρ = 1050 kg.m-3 ) and of viscosity 1.8 mPa s, with a concentration between 15 and 40%. The suspension is injected in glass Hele-Shaw cells with different apertures 0.4 < H < 1 mm and with several widths (5 mm < W < 12 mm) and lengths (100 < L < 300 mm) respectively parallel and perpendicular to the fluid motion. A programmable pump produces square wave flow rate variations with a period 0.4 < T < 9.6 s and peak to peak amplitudes of the fluid displacement 0.5 < A < 8 mm. The visualizations of the distribution of particles across the gap of the cell show that the bands traverse to the flow created by the instability correspond to spatial variations of the volume fraction ϕ of the particles in the bulk of the suspension. The instability occurs only for ϕ > 0.25 and in a limited range of flow rates depending on T. The instability is not observed in cells of aperture H lower than 10 particle diameters (no instability occurred in a Hele-Shaw cell of aperture H =0.4 mm with a suspension of particles of diameter d = 60 µm and for H =0.3 mm with d = 40 µm). We checked that the instability does not result from the influence of sedimentation/buoyancy on the particles by varying the orientation of the cell with respect to the vertical. The characteristic wavelength λ of the band system has been determined by image analysis in the center part of the cell width: λ increases significantly with the opening H of the cell (from 0.8 to 2 mm) as H varies from 0.4 to 1 mm. In contrast, λ does not seem to depend on W or of L. Moreover, λ varies weakly with T and increases only slowly with A. Period doubling is frequently observed in the spacing of the bands due to a difference between the distances of the bands from their two nearest neighbors.