Colloidal systems provide excellent opportunities for insight into fundamental mechanisms of phase transitions, which include the glass transition [1], crystal nucleation [2], and growth [3]. In atomic systems, the timescale for phase transitions is extremely small and it is usually very difficult to observe dynamically these processes, whereas micron-sized colloidal particles aggregate on timescales that are many orders-of-magnitude longer and are therefore readily observable.

A long-standing aim of colloidal crystallization research has been the fabrication of novel or useful crystal structures that may have applications as templates for photonic band-gap structures. The quality and morphology of these crystal structures are strongly influenced by their interfacial growth dynamics, which depend on the system volume fraction and the nature of the inter-particle interaction. Several types of interaction systems have been used to control crystal nucleation and growth, ranging from simple hard spheres (purely entropic) to electrostatic interactions to tunable short-range attractions. The latter recently has been realized experimentally using single-stranded DNA brushes grafted on polystyrene micron-sized particles [4] and an effective pair potential was measured [5], which can be used as input to simulations. This interaction is tunable by temperature and DNA sequence and length.

Recently, binary “A-B” solid-solution crystals have been grown in which different A-A, A-B, and B-B interactions were engineered using DNA sequence mismatches. Binary crystallization offers a detailed probe of segregation behavior because the crystal composition can be accurately measured experimentally. In this talk, Monte Carlo techniques are used to probe the mechanisms of segregation at the interface of growing binary crystals and the results are compared with experimental measurements. It is shown that a sequence of different microscopic processes with different equilibration times is responsible for setting the observed segregation coefficient. For weakly driven systems, particle attachment and detachment from the crystal surface controls segregation, while for strongly driven growth, diffusion away from the growing crystal front becomes the controlling phenomenon.

[1] E.R. Weeks, J.C. Crocker, A.C. Levitt, A. Schofield, and D.A. Weitz, Science 287, 627 (2000). [2] P.N. Pusey and W.v. Megen, Nature 320, 340 (1986). [3] U. Gasser, E.R. Weeks, A. Schofield, P.N. Pusey, and D.A. Weitz, Science 292, 258 (2001). [4] A.J. Kim, P.L. Biancaniello, and J.C. Crocker, Langmuir. 22, 1991 (2006). [5] P.L. Biancaniello, A.J. Kim, and J.C. Crocker, Phys. Rev. Lett. 94, 058302 (2005).