Laser-induced fluorescence is used to image the central plane of the m
ix region of two immiscible liquids subject to the Rayleigh-Taylor ins
tability at an acceleration of 70 times Earth's gravity. The size of t
he mixing zone grows with coefficient alpha(b) greater than or similar
to 0.054. Individual bubbles are clearly resolved and their structure
is measured. The widths of the bubbles are narrower than recent model
s predict. An internal structure in the bubbles appears after the mixi
ng zone is established. This structure suggests a cascade to small wav
elengths. The cascade is required for molecular mix.