Tricritical codimension-two point near the onset of convection in viscoelastic liquids

Abstract
We discuss the onset of convection in viscoelastic liquids. We find for the first time that for some value of the parameters the tricritical point of one of the instability branches coincides with the codimension two point. Such a phenomenon seems to have no analog in systems close to thermodynamic equilibrium. In addition we predict that the oscillatory instability sets in as finite-amplitude traveling waves for most Prandtl numbers.