Dynamics and structure of dense dusty plasmas using 3D tomographic imaging*

When levitated at the edge of an argon plasma sheath, micron-sized dust particles can form three-dimensional structures when the confinement is sufficiently strong. Depending on the neutral gas pressure and rf power, the particles can form and coexist in different phases: a low temperature crystal or a high temperature liquid. Here we report how a structural transition can occur between a 2D crystal and a 3D, complex liquid phase. Our experiments utilize a permanent magnetic field to enhance confinement, and the complex liquid phase is characterized by strong, non-reciprocal interactions between vertical layers due to an ion wake in the plasma sheath. Particles often form vertical pairs or triplets that resemble strings. This transition between phases can be quantified by the pair correlation function and the distribution of inter-particle distances. Importantly, the 3D tomographic imaging of our system using a rapidly scanning laser sheet allows us to simultaneously characterize the individual position and velocity of thousands of particles, which is important since we find there are strong gradients in particle density and temperature within the sheath.  By coarse-graining the above quantities, more statistical characteristics in the complex fluid phase can be identified and compared to other soft matter systems.