More than twenty years ago, Sparrow and Husar (1969) investigated experimentally in water the buoyancy-induced instability of the free convection boundary layer that forms on the upper surface of a constant temperature, heated flat plate which is inclined at an angle θ relative to the vertical. The flow was visualized by an electrochemical technique using thymol blue, a pH indicator. Dark dye formed at the surface of the plate, which served as the negative electrode, and was swept towards regions of upwelling from the plate when Rayleigh-Benard convection occurred.
