Simiso K. Mkhonta

Exploring the complex world of two-dimensional ordering with three modes

The world of two-dimensional crystals is of great significance for the design and study of structural and functional materials with novel properties. Here we examine the mechanisms governing the formation and dynamics of these crystalline or polycrystalline states and their elastic and plastic properties by constructing a generic multimode phase field crystal model. Our results demonstrate that a system with three competing length scales can order into all five Bravais lattices, and other more complex structures including honeycomb, kagome, and other hybrid phases. In addition, nonequilibrium phase transitions are examined to illustrate the complex phase behavior described by the model. This model provides a systematic path to predict the influence of lattice symmetry on both the structure and dynamics of crystalline and defected systems.

Grain boundary properties and collective dynamics of inversion domains in binary two-dimensional materials

Understanding and controlling the properties and dynamics of topological defects is a lasting challenge in the study of two-dimensional materials, and is crucial to achieve high-quality films required for technological applications. Here grain boundary structures, energies, and dynamics of binary two-dimensional materials are investigated through the development of a phase field crystal model that is parametrized to match the ordering, symmetry, energy, and length scales of hexagonal boron nitride. Our studies reveal some new dislocation core structures for various symmetrically and asymmetrically tilted grain boundaries, in addition to those obtained in previous experiments and first-principles calculations. We also identify a defect-mediated growth dynamics for inversion domains governed by the collective atomic migration and defect core transformation at grain boundaries and junctions, a process that is related to inversion symmetry breaking in binary lattice.

Emergence of Chirality from Isotropic Interactions of Three Length Scales

Chirality is known to play a pivotal role in determining material properties and functionalities. However, it remains a great challenge to understand and control the emergence of chirality and the related enantioselective process particularly when the building components of the system are achiral. Here we explore the generic mechanisms driving the formation of two-dimensional chiral structures in systems characterized by isotropic interactions and three competing length scales. We demonstrate that starting from isotropic and rotationally invariant interactions, a variety of chiral ordered patterns and superlattices with anisotropic but achiral units can self-assemble. The mechanisms for selecting specific states are related to the length-scale coupling and the selection of resonant density wave vectors. Sample phase diagrams and chiral elastic properties are identified. These findings provide a viable route for predicting chiral phases and selecting the desired handedness.

Soft elasticity in solids composed of ellipse-shaped particles

We present a method for studying the influence of internal rotational degrees of freedom on the elastic properties of crystals composed of ellipsoidal particles. We derive the conditions under which a stretched-triangular lattice of ellipsoidal particles can exhibit a vanishing shear modulus. Analytical predictions are confirmed with numerical calculations. Numerical results also show that internal rotational modes can delay the proliferation of dislocations in the plastic regime.

Novel mechanical properties in lamellar phases of liquid-crystalline diblock copolymers

Abstract.Structural properties of flexible nematic diblock copolymers in the lamellar phase are investigated using a mean-field model. We address two complementary questions on the mechanics of the system: 1) How does the nematic order affect the elasticity of the one-dimensional solid? 2) What effect does the block copolymer microstructure has on the orientation of the nematic director? In the limit when the microstructure does not influence the nematic director orientation we predict a soft lamellar compression mode. When the microstructure does influence the nematic director orientation, small compressions lead to conventional elasticity, until a critical strain is reached, where there is then a transition to a softer response. On the other hand, we show that an identifiable lamellar symmetry provides a direction along which the nematic director prefers to align. Our model provides avenues to explore nonlinear properties of flexible diblock copolymers in which the monomers on both sides have mesogenic side groups.

Models for Calculating Monthly Average Solar Radiation from Air Temperature in Swaziland

Solar radiation is an important energy source for mankind. Accurate data of solar radiation levels for a particular location is vital for optimum operation of solar energy transducers such as photovoltaic cells and solar thermal collectors. In this work, it is shown a linear relationship exist between monthly average temperatures and solar radiation in Swaziland. The correlation has been utilized to develop two mathematical models for the estimation of solar radiation: one from the measured monthly average temperatures and the other from the square-root of the difference between measured maximum and minimum monthly average temperatures. Both models fit the data well and can be applied to estimate solar radiation in other parts of the region.