Mixed wave mode: Direct evidence of phase transition during uphill-diffusion-based regular pattern formation

The formation of uphill-diffusion-based regular patterns in thin films of solutions was reported two years ago, which was explained as phase transition under non-equilibrium conditions in spinodal decomposition systems. However, definitive evidence is still required to confirm further that the regular patterns are produced via phase transition indeed rather than anisotropic spinodal decomposition. In this work, we observed that an ordered harmonic chemical wave and random spinodal wave can be simultaneously formed in polymer semiconductor solution. Such a mixed wave mode offers clear evidence that the ordered patterns originated from phase transition at non-equilibrium conditions.