Patterns of fluid intrusion in a Hele-Shaw cell of visco-elasto-plastic media

Fracturing from plastic failure and viscous fingering can occur simultaneously in many geological and engineering systems, but no model describes the possible transitions and feedbacks between the two. To explore fingering and fracturing we access the visco-plastic-elastic transition with injections in a weak transparent gel. We create a tunable reaction in Carbopol 934 gel, a well-known Hershell-Bulkely fluid, by changing the pH and mixture to control the cross-linking in the gel, and hence the stiffness and yield-stress. We then inject the gel in a Hele-Shaw cell and inject a glycerine solution creating a fingering instability. The gel has a yield stress and shear-thinning behavior, and therefore the plastic instabilities, viscous stresses, surface tension and elastic response of the gel work to create a phase space with a rich and complex interplay of fingering and fracture like dynamics. Our experiments have applications in geosciences by illuminating geometric patterns in show blunted tips and elastic like fracture that might be seen in rock veining and producing escape-type structures that might impact carbon sequestration efforts in the field. Such failure phenomena may suggest a more viscous type intrusion rather than sharp fracture singularities.