Crossover from critical to tricritical behavior in a nonequilibrium system: The convective instability in a binary fluid mixture

We derive an amplitude equation for the convective instability of a binary fluid mixture in a porous medium near the nonequilibrium analog of a tricritical point. We find that the prefactor of the quintic term in this equation is always stabilizing in the vicinity of the "tricritical" point. Therefore, it seems posible to observe experimentally (e.g., via measurement of the Nusselt number) the crossover from classical-mean-field to tricritical behavior.