Viscous fingering patterns of silica suspensions in polymer solutions: effects of viscoelasticity and gravity

Viscous fingering patterns of silica suspensions in polymer solutions, where polymer chains are adsorbed on silica particles, were investigated by pushing air or nitrogen gas in the radial Hele-Shaw cell. Fingering patterns of the stable silica suspensions showed a systematic change from tip-splitting to side branched patterns with an increase in free polymer concentration in suspended media, irrespective of silica content. Such changes in the pattern morphology were similar to those of polymer solutions with the corresponding concentration. The finger growth rate almost increased with a power law of the ratio of the injection pressure and effective shear viscosity, but the exponent was in disagreement with Darcyʹs law modified by taking into account shear thinning effects. The finger width was followed with the power law of the elasticity of the silica suspension, but the resulting exponent was much larger than that predicated by existing theory. The fingering pattern of the unstable silica suspensions under terrestrial gravity conditions was the tip-splitting type, whereas under microgravity conditions the tip-splitting pattern was slightly suppressed and the fingers were narrower, indicating that sedimentation no longer occurred. For the stable silica suspensions the tip-splitting pattern was much suppressed and the side-branching appeared, irrespective of the gravity conditions. Furthermore, the finger growth rate in microgravity conditions was somewhat faster than that in normal conditions due to less convection induced by gravity.