|dc.description.abstract||The microscopic liquid can be treated as a strongly coupled many-body system, which exhibits vast and complicated micro-dynamics. It has been a hot topic in the past decade. Nevertheless, the previous studies are mainly limited to the system with isotropic coupling. It is very interesting to extend the investigation to the liquid with anisotropic coupling. 2+1D liquids composed of bundles of flexible chains is one of the simplest examples, where the coupling along the chain direction is stronger than that in the transverse plane. Due to the lack of direct experimental observation, the micro-picture of 2+1D liquid remains elusive.
The dusty plasma liquid formed by negatively charged dust particle suspended in low pressure gaseous discharges, where the dust particle interacts with each other through the Coulomb interaction, provides us a platform to mimic and understand the generic microscopic dynamical behaviors of liquids at the kinetic level because of the capability of direct visualization. The wake field of downward ion wind on particles at larger diameter (> 3 mum) provides extra vertical coupling and alignment, and induces the chain bundle structure. When the chain length is long (40 particle per chain), the chains are flexible. The 2+1D dusty plasma liquid is then formed.
In this work, the micro-structure and motion in the cold 2+1D dusty plasma liquid were experimentally investigated through our stereo scanning dust tracking system. It is found that the horizontal structure and motion are similar to those of the 2D liquids. When the observation time is shorter than the relaxation time of the system, particles can exhibit collective motion, which is also associated with anomalous diffusion. Horizontally, the collective motion can be classified to the longitudinal and the transverse types. Along the vertical chain, under the suppression of the vertical flipping, only the transverse type collective motion is allowed. The cooperation of horizontal and vertical collective motion leads to the special collective stereo excitations, such as straight vertical chains with small amplitude wiggling, chain tilting-restraightening, bundle twisting-restraightening, and chain breaking-reconnection. The demonstration of the first direct visualization of those basic excitations was conducted in this work. In addition, horizontal micro-structure and the stereo collective excitations are found to be statistically correlated through the measurement of correlation probability.