DIFFUSION-LIMITED AGGREGATION AND VISCOUS FINGERING IN A WEDGE - EVIDENCE FOR A CRITICAL ANGLE

We show that both analytic and numerical evidence points to the existe nce of a critical angle of eta approximate to 60 degrees-70 degrees in viscous fingers and diffusion-limited aggregates growing in a wedge. The significance.of this angle is that it is the typical angular sprea d of a major finger. For wedges with an angle larger than 2 eta, two f ingers can coexist. Thus a finger with this angular spread is a kind o f building block for viscous fingering patterns and diffusion-limited aggregation clusters in radial geometry.