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

• Received 15 October 1997

DOI:https://doi.org/10.1103/PhysRevE.57.6913