Dynamics of Long-Wavelength Fluctuations in a Fluid Layer Heated from Above

We report an experimental study of the dynamics of thermal fluctuations in a 4.86 mm thick layer of CS₂ heated from above. Stabilizing gradients ranged from 10.3 to 61.7 K/cm. Power spectral measurements were made over the wave vector range 9 cm^-1<q<100 cm^-1, using a shadowgraph apparatus. Over this range, the spectra change from having a well-resolved oscillatory mode at low q to monotonic decay at higher q. For the smallest gradient, the results for the power spectra compare very well with theory, but slight deviations are seen for larger gradients.