Two-dimensional colloidal systems in time-dependent magnetic fields How to define crystallinity in 2D on a local scale

We use super-paramagnetic colloidal particles confined by gravitation to a flat water-air interface as a model system to study the non-equilibrium liquid-solid phase transition in two dimensions. The system temperature is adjustable by changing the strength of an external magnetic field perpendicular to the water-air interface. Increasing the magnetic field on a timescale of milliseconds quenches the liquid to a strongly super-cooled state. If the system is cooled down out of equilibrium the solidification differs drastically from the equilibrium melting and freezing scenario as no hexatic phase is observable. The system solidifies to a polycrystalline structure with many grains of different orientations. Since the local closed packed order in two dimensions is sixfold, in both the fluid and the crystalline state, sensitive measures have to be developed. In the present manuscript we compare different methods to identify crystalline cluster locally and motivate the threshold values. Those are chosen in comparison with the isotropic fluid on one hand and large mono-crystals in thermal equilibrium on the other hand. With the given criteria for crystalline cluster the cluster are found not to be circular and fractal dimensions of the grains are given.