Symmetry breaking in a fixed circle and in a free-boundary problem.

(a) Instability of a symmetric steady state of ZV model in a fixed circle: snapshots of dimensionless myosin density (pseudo-color) and actin velocities (arrows) at specified times t after myosin was slightly shifted left of center; computations were done for μ_tot = 1.5π and α = 0.5. (b) Transition to unidirectional motility in ZS model; dimensionless myosin concentration (pseudo-colors) and boundary velocities (arrows) are shown for the solution obtained with α = 1, v₀ = 5, and μ_tot = 1.5π; the cell assumes steady unidirectional motility after t = 14 (S2 Movie).