Multicriticality in viscoelastic fluids heated from below

Phase diagrams near codimension two (CT) bifurcation points are analyzed. We consider CT points at which two distinct modes become unstable simultaneously. Such a point is expected to occur in a layer of Maxwell fluid heated from below, where one mode is oscillatory and the other one is stationary. We find that the phase diagram exhibits a novel mixed phase in which both stationary and oscillatory modes are present. Moreover, depending on the coupling between the two modes the system may exhibit a direct transition from the conductive to the mixed phase.