Quantitative Metrics for Assessing Positional and Orientational Order in Colloidal Crystals.

Although there are numerous self-assembly techniques to prepare colloidal crystals, there is great variability in the methods used to characterize order and disorder in these materials. We assess different kinds of structural order from more than 70 two-dimensional microscopy images of colloidal crystals produced by many common methods, including spin-coating, dip-coating, convective assembly, electrophoretic assembly, and sedimentation. Our suite of analysis methods includes measures for both positional and orientational order. The benchmarks are two-dimensional lattices that we simulated with different degrees of controlled disorder. We find that translational measures are adequate for characterizing small deviations from perfect order, whereas orientational measures are more informative for polycrystalline and highly disordered crystals. Our analysis presents a unified strategy for comparing structural order among different colloidal crystals and establishes benchmarks for future studies.

Inverse melting in a two-dimensional off-lattice model.

We carry out computer simulations of a simple, two-dimensional off-lattice model that exhibits inverse melting. The monodisperse system comprises core-softened disks interacting through a repulsive square shoulder located inside an attractive square well. By systematically varying the potential parameters, we increase the pressure range over which the liquid freezes to a crystal upon isobaric heating. The effect is largely controlled by the extent of the shoulder. Despite occurring in two dimensions, the melting transition is first order and to a liquid, rather than to a hexatic or quasicrystal phase. We also provide comment on a commonly employed correlation function used to determine the degree of translational ordering in a system.

Phase diagram of a two-dimensional system with anomalous liquid properties.

Using Monte Carlo simulation techniques, we calculate the phase diagram for a square-shoulder square-well potential in two dimensions that has been previously shown to exhibit liquid anomalies consistent with a metastable liquid-liquid critical point. We consider the liquid, gas, and five crystal phases, and find that all the melting lines are first order, despite a small range of metastability. One melting line exhibits a temperature maximum, as well as a pressure maximum that implies inverse melting over a small range in pressure.

Folding and binding energy of a calmodulin-binding cell antiproliferative peptide.

We carry out a computational study of a calmodulin-binding peptide shown to be effective in reducing cell proliferation. We find several folded states for two short variants of different length of the peptide and determine the location of the binding site on calmodulin, the binding free energy for the different conformers and structural details that play a role in optimal binding. Binding to a hydrophobic pocket in calmodulin occurs via an anchoring phenylalanine residue of the natively disordered peptide, and is enhanced when a neighbouring hydrophobic residue acts as a co-anchor. The shorter sequence possesses better binding to calmodulin, which is encouraging in terms of the development of non-peptide analogues as therapeutic agents.

Simulation of a two-dimensional model for colloids in a uniaxial electric field.

We perform Monte Carlo simulations of a simplified two-dimensional model for colloidal hard spheres in an external uniaxial ac electric field. Experimentally, the external field induces dipole moments in the colloidal particles, which in turn form chains. We therefore approximate the system as composed of well-formed chains of dipolar hard spheres of a uniform length. The dipolar interaction between colloidal spheres gives rise to an effective interaction between the chains, which we treat as disks in a plane, that includes a short-range attraction and long-range repulsion. Hence, the system favors finite clustering over bulk phase separation, and indeed we observe at low temperature and density that the system does form a cluster phase. As the density increases, percolation is accompanied by a pressure anomaly. The percolated phase, despite being composed of connected, locally crystalline domains, does not bear the typical signatures of a hexatic phase. At very low densities, we find no indication of a "void phase" with a cellular structure seen recently in experiments.