Fracture processes are ubiquitous in soft materials, even in complex fluids,
subjected to stresses. To investigate these processes in a simple geometry, we
use a model self-assembled transient gel and study the instability patterns
obtained in a radial Hele-Shaw cell when a low viscosity oil pushes the more
viscous transient gel. Thanks to an analysis of the morphology of the patterns,
we find a discontinuous transition between the standard Saffman-Taylor
fingering instability and a fracturing instability as the oil injection rate
increases. Our data suggest that the flow properties of the gel ahead of the
finger tip controls the transition towards fracturing. By analyzing the
displacement field of the gel in the vicinity of the fingers and cracks, we
show that in the fingering regime, the oil gently pushes the gel, whereas in
the fracturing regime, the crack tears apart the gel, resulting in a strong
drop of the gel velocity ahead of the crack tip as compared to the tip
velocity. We find a unique behavior for the whole displacement field of a gel
around a crack, which is drastically different from that around a finger, and
reveals the solid-like behavior of the gel at short time. Our experiments and
analysis provide quantitative yet simple tools to unambiguously discriminate a
finger from a crack in a visco-elastic material.Comment: to appear in Soft Matte
