Liquid crystals boojum-colloids

Colloidal particles dispersed in a liquid crystal lead to distortions of the director field. The distortions are responsible for long-range effective colloidal interactions whose asymptotic behaviour is well understood. The short distance behaviour of the interaction, however, is sensitive to the structure and dynamics of the topological defects nucleated near the colloidal particles in the strong anchoring regime. The full non-linear theory is required in order to determine the interaction at short separations. Spherical colloidal particles with sufficiently strong planar degenerate anchoring nucleate a pair of antipodal surface topological defects, known as boojums. We use the Landau-de Gennes formalism in order to resolve the mesoscopic structure of the boojum cores and to determine the pairwise colloidal interaction. We compare the results in three (3D) and two (2D) spatial dimensions. The corresponding free energy functionals are minimized numerically using finite elements with adaptive meshes. Boojums are always point-like in 2D, but acquire a rather complex structure in 3D which depends on the combination of the anchoring potential, the radius of the colloid, the temperature and the LC elastic anisotropy. We identify three types of defect cores in 3D which we call single, double and split core boojums, and investigate the associated structural transitions. In the presence of two colloidal particles there are substantial re-arrangements of the defects at short distances, both in 3D and 2D. These re-arrangements lead to qualitative changes in the force-distance profile when compared to the asymptotic quadrupole-quadrupole interaction. In line with the experimental results, the presence of the defects prevents coalescence of the colloidal particles in 2D, but not in 3D systems.Comment: 18 pages, 21 figure

Three-dimensional patchy lattice model: ring formation and phase separation

We investigate the structural and thermodynamic properties of a model of particles with $2$ patches of type $A$ and $10$ patches of type $B$. Particles are placed on the sites of a face centered cubic lattice with the patches oriented along the nearest neighbor directions. The competition between the self-assembly of chains, rings and networks on the phase diagram is investigated by carrying out a systematic investigation of this class of models, using an extension of Wertheim's theory for associating fluids and Monte Carlo numerical simulations. We varied the ratio $r\equiv\epsilon_{AB}/\epsilon_{AA}$ of the interaction between patches $A$ and $B$, $\epsilon_{AB}$, and between $A$ patches, $\epsilon_{AA}$ ($\epsilon_{BB}$ is set to $0$) as well as the relative position of the $A$ patches, i.e., the angle $\theta$ between the (lattice) directions of the $A$ patches. We found that both $r$ and $\theta$ ($60^\circ,90^\circ,$ or $120^\circ$) have a profound effect on the phase diagram. In the empty fluid regime ($r < 1/2$) the phase diagram is re-entrant with a closed miscibility loop. The region around the lower critical point exhibits unusual structural and thermodynamic behavior determined by the presence of relatively short rings. The agreement between the results of theory and simulation is excellent for $\theta=120^\circ$ but deteriorates as $\theta$ decreases, revealing the need for new theoretical approaches to describe the structure and thermodynamics of systems dominated by small rings.Comment: 26 pages, 10 figure

Colloidal interactions in two dimensional nematics

The interaction between two disks immersed in a 2D nematic is investigated (i) analitically using the tensor order parameter formalism for the nematic configuration around isolated disks and (ii) numerically using finite element methods with adaptive meshing to minimize the corresponding Landau-de Gennes free energy. For strong homeotropic anchoring, each disk generates a pair of defects with one-half topological charge responsible for the 2D quadrupolar interaction between the disks at large distances. At short distance, the position of the defects may change, leading to unexpected complex interactions with the quadrupolar repulsive interactions becoming attractive. This short range attraction in all directions is still anisotropic. As the distance between the disks decreases their preferred relative orientation with respect to the far-field nematic director changes from oblique to perpendicular.Comment: 7 pages, 7 figure

Language of instruction: A critical aspect of epistemological access to higher education in South Africa

The article discusses language as a critical aspect of epistemological access to higher education in South Africa through a critical analysis of students’ views on language preferences and attitudes at Durban University of Technology (DUT). Language of instruction at higher education institutions is one amongst a plethora of challenges faced in providing equity in higher education in a socially divided post-apartheid South Africa. This discussion of language and access takes place within broader epistemological access challenges faced in South African higher education. The data generated by 45 semi-structured interviews with undergraduate students from six faculties was collected as part of an institution-wide “Who are our students?” research project. The phenomenological thematic analysis arose from the three research questions asked: what is your home language and what language would you prefer to be taught in and why? Some students did not provide an explanation for their choice as they felt it was self-evident, while some provided interesting reasons for their choices. A critical analysis of the findings using Morrow’s epistemological access theory is conducted against the backdrop of the context of English dominance over indigenous languages; the history of South African higher education, current DUT and higher education policy as it pertains to language and the subsequent challenges around equitable access