Nonperiodic time dependence at the onset of convection in a binary liquid mixture

We report measurements of the convective heat transport near the onset of convection in a normal ³He-⁴He mixture for values of the separation ratio ψ between -0.044 and -0.010. For ψ-0.018, the first bifurcation was to a relatively large-amplitude time-dependent state with characteristic frequencies over an order of magnitude smaller than predicted by linear-stability analysis. The onset of the time dependence was intermittent and not hysteretic. Above onset, the frequency spectrum of the time dependence either contained discrete frequencies or not, depending on ψ.