Two-frequency states at the onset of convection in binary mixtures. .

Oscillatory convection in binary fluid mixtures takes the form of traveling waves. We show that the wavenumber of the resulting flow satisfies a resonance condition in a cell of finite length, in analogy with light in a laser cavity. By tuning the length of the convection cell so that two adjacent modes have approximately the same growth rate, we have been able to produce two-frequency convective flows at onset. These flows are modulated in space and time due to the interference of the two modes. Study of these modulated states has allowed accurate determination the linear parameters of the system.