Freezing of a 2D colloidal system in the presence of induced disorder

Abstract

We investigated the thermodynamics of two-dimensional colloidal systems in the presence of induced disorder. The disorder was generated by confining the colloids on a substrate with slowly varying modulations which act, due to gravity, as a repulsive and random potential. While freezing the systems, we tracked the particle positions in equilibrium states and studied the structure and dynamic as a function of disorder to investigate the phase behavior. Therefore, orientational correlation functions in space and time were analyzed as well as a modified 2D dynamical Lindemann parameter. Since the behavior of those functions indicated differences to the undisturbed system, snapshots of particle positions and local director fields were discussed in real space.We showed that the first occurrence of hexatic characteristics as well as the transition to a crystalline state are, triggered by the induced disorder, shifted to lower temperatures which is in agreement with theories by Nelson [1], and Cha and Fertig [2]. Further, we found that, at finite disorder and high interaction strengths, the system occupies a solid state. This verifies simulations by Herrera-Velarde and von Grünberg [3], as well as Cha and Fertig's reconsiderations of Nelson's theory. The latter postulates a re-entrance from the solid to the hexatic phase at low temperatures for arbitrary small disorder strengths what was not seen in our experiment. In addition, we observed the developing of phase equilibria indicating first-order characteristics which also seem to be induced by disorder and showed that their range is also dependent on the disorder strength.[1] D. Nelson, Physical Review B 27(5), 2902 (1983).[2] M. Cha and H. Fertig, Physical Review Letters 74(24), 4867 (1995).[3] S. Herrera-Velarde and H. von Grünberg, Soft Matter 5, 391 (2008).