Annihilation of nematic point defects : Pre-collision and post-collision evolution

• Published in: The European physical journal. E, Soft matter and biological physics Soft matter and biological physics, 2006-05, Vol.20 (1), p.71-79
• Main Authors: SVETEC, M, KRALJ, S, BRADAC, Z, ZUMER, S
• Format: Article
• Language: English
• Subjects: Anisotropy; Computer Simulation; Condensed matter: structure, mechanical and thermal properties; Elasticity; Exact sciences and technology; Liquid crystals; Liquid Crystals - chemistry; Models, Chemical; Physics; Structure of solids and liquids; crystallography
• ISSN: 1292-8941; 1292-895X
• DOI: 10.1140/epje/i2005-10120-9

The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied. The semi-microscopic lattice-type model and Brownian molecular dynamics are used. We distinguish among the i) early pre-collision, ii) late pre-collision, iii) early post-collision, and iv) late post-collision stages. In the pre-collision stage our results agree qualitatively with the existing experimental observations and also continuum-type simulations. The core of each defect exhibits a ring-like structure, where the ring axis is set perpendicular to the cylinder symmetry axis. For xi(0)d/(2R) > 1 the interaction between defects is negligible, where xi(0)d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for xi(0)d/(2R) < 1. For close enough defects their separation scales as xi(d) [see text] (t(c)- t)(0.4+/-0.1), where t(c) stands for the collision time. In elastically anisotropic medium the hedgehog is faster than the anti-hedgehog. In the early pre-collision stage the defects can be treated as point-like particles, possessing inherent core structure, that interact via the nematic director field. In the late pre-collision stage the cores reflect the interaction between defects. After the collision a charge-less ring structure is first formed. In the early post-collision stage the ring adopts an essentially untwisted circular structure of the radius xi(r). In the late post-collision stage we observe two qualitatively different scenarios. For mu = xi(r)/R < mu(c) approximately 0.25 the ring collapses leading to the escaped radial equilibrium structure. For mu > mu(c) the chargeless ring triggers the nucleation growth into the planar polar structure with line defects.