Periodic spinodal decomposition in a binary liquid mixture

When a binary mixture is periodically driven through its consolute point, two distinct regimes are found. In one regime the system ultimately separates into two phases; in the other regime the system remains in a disordered single phase. This transition has been studied by light scattering in a critical mixture of isobutyric acid and water. The system behaves differently in these two regimes, though a characteristic spinodal ring is observed in both of them. In the disordered phase very strong composition fluctuations appear. In the ordered phase there is clear evidence of mean-field behavior. The structure factor S(k,t) exhibits a self-similarity in both regimes, but its scaling form is different. In the ordered state, S(k,t)∼1/$k^4$ for large wave number k. This implies that the scattering is dominated by the sharp interfaces between the domains. In the disordered state, S(k,t) exhibits a weaker dependence on k. Where possible the measurements are compared with a theory of periodic spinodal decomposition by Onuki. The number of parameters in the theory is insufficient to explain all of our measurements.