Stripes instability of an oscillating non-Brownian iso-dense suspension of spheres

ROHT, Y. L.; IPPOLITO, I.; HULIN, J. P.; SALIN, D.; GAUTHIER, G.

EUROPHYSICS LETTERS

EPL ASSOCIATION

Año: 2018 vol. 121

0295-5075

We analyze experimentally the behavior of a non-Brownian, iso-dense suspension ofspheres submitted to periodic square wave oscillations of the flow in a Hele-Shaw cell of gap H.Wedo observe an instability of the initially homogeneous concentration in the form of concentrationvariation stripes transverse to the flow. The wavelength of these regular spatial structures scalesroughly as the gap of the cell and is independent of the particle concentration and of the periodof oscillation. This instability requires large enough particle volume fractions φ ≥ 0.25 and a gaplarge enough compared to the sphere diameter (H/d ≥ 8). Mapping the domain of the existence ofthis instability in the space of the control parameters shows that it occurs only in a limited rangeof amplitudes of the fluid displacement. The analysis of the concentration distribution across thegap supports a scenario of particle migration towards the wall followed by an instability due toa particle concentration gradient with a larger concentration at the walls. In order to accountfor the main features of this stripes instability, we use the theory of longitudinal instability dueto normal stresses difference and recent observations of a dependence of the first normal stressesdifference on the particle concentration.