Rayleigh-Benard Convection in Pure and Binary Fluids

We describe flow-visualization studies of the onset of convection in ethanol-water mixtures for Psi in the range -0.56 to -0.05. Convection is triggered by exponentially growing oscillations at the frequency of the oscillatory instability with a growth rate which is linearly proportional to the Rayleigh number, R - Rco. These oscillations take the form of counter propagating, spatially-growing traveling waves.(1) However, the oscillations do not saturate. Instead, they directly trigger overturning convection in the form(2) of slowly-moving traveling waves. Previous observations of time dependence near the onset of convection in binary fluid mixtures will be discussed in terms of the traveling-wave states described here.