Radial fingering in viscoelastic media, an experimental study

We report measurements of two dimensional radial fingering in viscoelastic concentrated suspensions (pastes) of colloidal particles, in small surface tension conditions. The displacement efficiency, the average finger width and the fractal dimension for growth have been measured in a broad range of paste viscosity and driving pressure. The growth process appears to be fractal in all the experimental conditions, with fractal exponents, DG, covering the whole range from 1.3 at very small tip velocity to 2.0 at very high tip velocity. The displacement efficiency, 03B4, defined as the volume fraction of paste displaced by water when water flows out of the paste, is almost zero at very low velocity, and seems to tend toward an asymptotic value close to 0.25 at high velocity, 03B4 is almost entirely dominated by the velocity dependence of the tip-splitting cascade. The average finger width has very little influence. Revue Phys. Appl. 22 (1987) 241-252 AVRIL 1987,