CONVECTIVE INSTABILITY IN A FLUID MIXTURE HEATED FROM ABOVE

Convection patterns in ethanol-water mixtures with negative psi are st udied when the fluid is heated from above. Although the linear analysi s predicts that the instability occurs at zero wave number, a large wa ve number pattern is observed. The onset is supercritical with a thres hold that is experimentally indistinguishable from zero. The convectio n amplitude exhibits damped oscillations for sudden change in the forc ing parameter. At the constant Rayleigh number the patterns first coar sen, then exhibit growth of narrow plumes. The instability appears to be related to salt fingering.