Motion, relaxation dynamics, and diffusion processes in two-dimensional colloidal crystals confined between walls

摘要

The dynamical behavior of single-component two-dimensional colloidal crystals confined in a slit geometrynis studied by Langevin dynamics simulation of a simple model. The colloids are modeled as pointlike particles,ninteracting with the repulsive part of the Lennard-Jones potential, and the fluid molecules in the colloidalnsuspension are not explicitly considered. Considering a crystalline strip of triangular lattice structure with n = 30nrows, the (one-dimensional) walls confining the strip are chosen as two rigidly fixed crystalline rows at eachnside, commensurate with the lattice structure and, thus, stabilizing long-range order. The case when the spacingnbetween the walls is incommensurate with the ideal triangular lattice is also studied, where (due to a transitionnin the number of rows, n → n − 1) the confined crystal is incommensurate with the confining boundaries, andna soliton staircase forms along the walls. It is shown that mean-square displacements (MSDs) of particles as anfunction of time show an overshoot and then saturate at a horizontal plateau in the commensurate case, the valuenof the plateau being largest in the center of the strip. Conversely, when solitons are present, MSDs are largestnin the rows containing the solitons, and all MSDs do not settle down at well-defined plateaus in the directionnparallel to the boundaries, due to the lack of positional long-range order in ideal two-dimensional crystals. ThenMSDs of the solitons (which can be treated like quasiparticles at very low temperature) have also been studiednand their dynamics are found to be about an order of magnitude slower than that of the colloidal particlesnthemselves. Finally, transport of individual colloidal particles by diffusion processes is studied: both standardnvacancy-interstitial pair formation and cooperative ring rotation processes are identified. These processes requirenthermal activation, with activation energies of the order of 10Tm (Tm being the melting temperature of the crystal),nwhile the motions due to long-wavelength phonons decrease only linearly in temperature.