Dynamics of particle---turbulence interaction at the dissipative scales*

We present results of a novel phosphorescent tagging technique that is particularly suited to study particle-laden flows. Using phosphorescent droplets we probe the dynamics of particle--turbulence interaction at the dissipative length scales. We create a cloud of droplets within a chamber capable of generating homogeneous, isotropic turbulence with zero-mean flow. The droplets have Stokes number St$\:\sim 1$, and the flow is intensely turbulent, with Reynolds number Re$_\lambda\:\approx 500$. Using a frequency-tripled Nd:YAG laser, we can tag a variety of volumes, such as thin slabs or thin, pencil-like cylinders. The droplets in these volumes glow during a few Kolmogorov times. By tracking the fate of pencil-shaped clouds using a fast (5 kHz) camera, we come to the surprising conclusion that they disperse faster than fluid elements, with a spreading rate reaching a maximum at ${\rm St} \approx 2$. Sheets of tagged droplets display preferential concentration at work; we discuss statistical quantities that can capture these events.