Crystallization process of a three-dimensional complex plasma

Characteristic timescales and length scales for phase transitions of real materials are in ranges where a direct visualization is unfeasible. Therefore, model systems can be useful. Here, the crystallization process of a three-dimensional complex plasma under gravity conditions is considered where the system ranges up to a large extent into the bulk plasma. Time-resolved measurements exhibit the process down to a single-particle level. Primary clusters, consisting of particles in the solid state, grow vertically and, secondarily, horizontally. The box-counting method shows a fractal dimension of $d_f\approx 2.72$ for the clusters. This value gives a hint that the formation process is a combination of local epitaxial and diffusion-limited growth. The particle density and the interparticle distance to the nearest neighbor remain constant within the clusters during crystallization. All results are in good agreement with former observations of a single-particle layer.

Figure 1

The positions of particles in the solid (red/light) and liquid (blue/dark) state, 0, 66, 178, and 244 s (from top to bottom) after the crystallization process has begun. Due to their high thermal speed, the absolute position of the particles in the liquid state may have an error.

Figure 2

The relation between the minimum amount of boxes needed to cover the largest solid cluster $N\left(L\right)$ and the corresponding length of the box $L$. The scans are done at 178 s , 211 s, and 244 s after the start of crystallization.

Figure 3

The particle density (calculated by the inverse volume of the Voronoi cell [29]) and the distance to the nearest neighbor for particles within a cluster depending on the time after starting the crystallization. The standard deviation in each time step is given by the error bars.