The dispersion of microscopic molecular clouds in turbulence

We write thin lines (width 60 $\mu$m) and small dots (size 100 $\mu$m) in a strongly turbulent jet flow of air. These patterns are written through photosynthesis of NO molecules using focussed light from a UV excimer laser and are followed in time through laser-induced fluorescence. The size of the written objects is a few times the Kolmogorov length. The Lagrangian trajectories of these clouds can be used for velocimetry, but also to study the dispersion of molecular tracers in turbulence. Our molecular tracers have Schmidt number unity and momentum diffuses at the same rate as mass. Ultraviolet images of the clouds offer a unique view on small- scale mixing. The clouds are dispersed by turbulence and by molecular diffusion, resulting in highly intermittent concentration fluctuations after a few small eddy turnover times. Consequently, large-order structure functions of the fluctuation concentration have strongly anomalous inertial-range scaling exponents.