Viscous fingering shapes in concentrated particle suspension flow

We carried out visualization experiments on the Hele-Shaw flow of concentrated particle suspensions and examined the change in viscous fingering pattern with increasing the particle concentration in suspension. The less viscosity fluid invading the particle suspension in the Hele-Shaw cell was compressed air. The test fluids were four kinds of suspensions with different particle concentrations, and the steady shear viscosity of higher-concentration suspensions showed remarkable shear-thickening. In the flow images when compressed air was supplied at high pressures, we confirmed the transition of viscous fingering shape to a dendrite pattern with increasing the particle concentration. The transition to the dendrite pattern was closely related to the shear-thickening property of shear viscosity. Furthermore, the path of compressed air after the occurrence of the dendrite pattern meandered slightly without spreading and propagated through the suspension with repeated side branching. Such an air path appears to suggest local particle aggregation induced by particle migration during the invasion of compressed air.