Epifanio G. Virga

Universal fine structure of nematic hedgehogs

We study in a Landau-de Gennes approach the biaxial structure of a nematic point defect with topological charge M = + 1. We aim to illuminate the role of the confining boundaries in determining the fine structure of the defect. We show that there are different regimes associated with different values of the ratio between the typical size R of the region in space occupied by the material and the biaxial correlation length ξb. For R/ξb>20 the core structure is already qualitatively universal, that is, independent of the confining geometry, while also for R/ξb>200 any quantitative difference is unlikely to be detected.

Curvature control of valence on nematic shells

Within a two-dimensional Landau–de Gennes tensorial formalism we study the equilibrium configurations of nematic shells, thin films of nematic liquid crystal deposited on the boundary of colloidal particles of arbitrary shape enforcing a degenerate tangential anchoring on the nematic molecules. In this formalism, defects appear wherever a scalar order parameter vanishes. Their total number is the colloidal valence, as this is the number of molecular bridges that can bind every colloid to its peers. We show how the defect organization on a colloidal particle is affected by the Gaussian curvature of the colloidal boundary, to the point of changing its valence.

Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation

Abstract.We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range

Metastable nematic hedgehogs

\Delta

Adhesive borders of lipid membranes

TBP of liquid-crystalline (LC) blue phases. Using a mesoscopic Landau-de Gennes type approach we obtain that the defect core replacement (DCR) mechanism yields in the diluted regime

Drops of nematic liquid crystals

\Delta

Local stability for a general wetting functional

TBP(x)

Local stability of biaxial nematic phases between two cylinders

\propto

Evolution of vesicles subject to adhesion

1/(1 - xb) , where x stands for the concentration of NPs and b is a constant. Our calculations suggest that the DCR mechanism is efficient if a local NP environment resembles the core structure of disclinations, which represent the characteristic property of BP structures. These predictions are in line with high-resolution ac calorimetry and optical polarising microscopy experiments using the CE8 LC and CdSe or aerosil NPs. In mixtures with CdSe NPs of 3.5nm diameter and hydrophobic coating the BPIII stability range has been extended up to 20K. On the contrary, the effect of aerosil silica nanoparticles of 7.0nm diameter and hydrophilic coating is very weak.

Phase polarity in a ferrofluid monolayer of shifted-dipole spheres

For nematic liquid crystals, we study the local stability of a radial hedgehog against biaxial perturbations. Our analysis employs the Landau - de Gennes functional to describe the free energy stored in a ball, whose radius is a parameter of the model. We find that a radial hedgehog may be either unstable or metastable, depending on the values of the elastic constants. For unstable hedgehogs, we give an explicit expression for the radius of the ball within which the instability manifests itself: it can be interpreted as the size of the biaxial core of the defect; it is of the same order of magnitude as the radius of the disclination ring predicted by Penzenstadler and Trebins model. The metastable hedgehogs predicted by our model are the major novelty of the paper. They tell us that we may also expect truly uniaxial point defects, whose core contains no biaxial structure.