Fracturing due to fluid intrusion into viscoelastic materials

Fluid intrusion into viscoelastic materials is studied by changing the viscoelastic property of a model material (agar gel) from a viscous fluid to an elastic solid. Viscous fingering is observed in the agar gel with a low concentration of agar and single plane cracking are observed in the agar gel with a high concentration of agar in our experiments. At the middle range of agar concentration, a new kind of pattern growth called ``viscoelastic fracturing'' is observed due to two competitive mechanism (viscous flow and fracture). A phase diagram for pattern formation is obtained as a function of the fluid injection pressure and the concentration of agar. Fractal dimensions for these patterns are calculated. The fractal dimension for the viscoelastic fracturing patterns ranges from 1.85 to 1.95, which is considerably different from the value $D=1.68$ for diffusion-limited aggregation.