Crystallization Dynamics of a Single Layer Complex Plasma

Abstract

We report a series of complex (dusty) plasma experiments, aimed at the study of the detailed time evolution of the re-crystallisation process following a rapid quench of a two dimensional dust liquid. The experiments were accompanied by large-scale (million particle) molecular dynamics simulations, assuming Yukawa type inter-particle interaction. Both experiment and simulation show a ∝ t α (power law) dependence of the linear crystallite domain size as measured by the bondorder correlation length, translational correlation length, dislocation (defect) density, and a direct size measurement algorithm. The results show two stages of order formation: on short time-scales individual particle motion dominates; this is a fast process characterized by α = 0.93 ± 0.1. At longer time-scales, small crystallites undergo collective rearrangement, merging into bigger ones, resulting in a smaller exponent α = 0.38 ± 0.06.From the very beginning of laboratory complex (dusty) plasma research, one of the main motivating and promising features of these strongly coupled many-particle systems has been the possibility for modeling classical collective phenomena occurring in atomic matter on a size and time scale that allows direct observation at the particle level [1]. Experiments addressing e.g. transport phenomena in two dimensions (heat conductivity [2], viscosity [3], and self-diffusion [4]), dislocation dynamics [5], various melting scenarios [6][7][8][9], and some aspects of freezing [10,11] have already been carried out.The fact that a two-dimensional, hexagonal crystalline structure could easily develop in a laboratory complex plasma experiment was clear from the early days of the field [12], even though the theoretical background explaining crystallization in low dimensions [13] is still incomplete and the subject of ongoing debate [14].Here we study the phenomena of pattern formation occurring when a liquid is rapidly quenched to a solid. Our qualitative expectation is that the initial amorphous liquid structure should evolve toward a ground state crystal through the process of domain coarsening, i.e., merging initially formed small crystallites into fewer and bigger ones. To characterize this process we measured the average linear size of the crystallites and recorded their time evolution. Size measurement is based on: (i) the bondangular correlation length, (ii) the translational correlation length, (iii) the inverse defect fraction, and (iv) a direct size measurement algorithm similar to the standard "flood fill" method used in simple graphical tools.Ref.[15] reported a time dependence of the orientational correlation length in the form t α with α ≈ 1/4 measured in a two dimensional temperature quenched experiment in a single layer of spherical block copolymer microdomains in a thin film. Recent experiments with superparamagnetic particles (∝ 1/r 3 interaction) have shed new light on the dynamics of crystallite formation in colloidal systems resulting in a currently accepted value of α ≈ 0.3 [16]. Recent equilibrium mel...

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