PHASE DYNAMICS NEAR A PARITY-BREAKING INSTABILITY

In a directional viscous fingering experiment, a phase diffusion behav ior of the regular array of cells is demonstrated. At constant wave nu mber, the evolution of the phase diffusion coefficient D with the para meter of the instability epsilon is reported. This coefficient D is fo und to decrease at low epsilon values in agreement with the presence o f an Eckhaus instability, but to increase strongly at large epsilon va lues. This unusual increase of D is also present in the analytical and numerical study of two coupled amplitude equations for the modes k an d 2k. The phase diffusion coefficient is found to diverge at the thres hold of the parity-breaking bifurcation.