In their numerical investigation of the two-dimensional growth of a pair of needlelike viscous fingers, Peterson and Ferry [Phys. Rev. A 39, 2740 (1989)] discovered that when the angle between the two needles is less than 0.920 154 rad, then a ‘‘shadowing’’ transition occurs, with one needle growing longer than the other. I show that this is a standard pitchfork bifurcation, with the critical angle being exactly (1- √2 /2)π.

• Received 25 February 1991

DOI:https://doi.org/10.1103/PhysRevA.43.7066