Model illustration. Three arms of length l i with local polymerization speed v i ( i = 1, 2 or 3) move along the confinement of a three-way symmetric Y-junction (black lines). All arms are connected by a spring. Green and yellow gradients denote areas of low or high concentration, respectively, of a polarity cue that inhibits the actin polymerization at the leading edges of the arms (red denotes the polymerization strength). At each edge, the friction force acts as a drag friction when the arm extends and as a slip-bond friction when the arm retracts 20 .
