From viscous fingering to viscoelastic fracturing in colloidal fluids

Fluid-displacement experiments in Hele-Shaw cells filled with a viscoelastic fluid show a novel transition between a viscous fingering (VF) regime producing fractal patterns of ``fingers'' and a viscoelastic facturing (VEF) regime producing fractal patterns of ``cracks.'' VEF patterns are characterized by branching angles of 90^∘{} with respect to the main crack, behind the tip, and by a lower fractal dimension than VF. The transition is controlled by several parameters, including the Deborah number and the system deformability.