Field-driven dynamics of nematic microcapillaries

Abstract

Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of study for their unique electro-optical properties which have resulted in various applications such as switchable (transparent/translucent) windows. These composites are manufactured using desirable "bottomup" techniques, such as phase separation of a liquid crystal/polymer mixture, which enable production of PDLC films at very large scales. LC domains within PDLCs are typically spheroidal, as opposed to rectangular for an LCD panel, and thus exhibit substantially different behaviour in the presence of an external field. The fundamental difference between spheroidal and rectangular nematic domains is that the former results in the presence of nanoscale orientational defects in LC order while the latter does not. Progress in the development and optimization of PDLC electrooptical properties has progressed at a relatively slow pace due to this increased complexity. In this work, continuum simulations are performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. Using a simplified elliptic cylinder (microcapillary) geometry as an approximation of spheroidal PDLC domains, the effects of geometry (aspect ratio), surface anchoring, and external field strength are studied through the use of the Landau-de Gennes model of the nematic LC phase. * nmabukhdeir@uwaterloo.ca;

Formation and field-driven dynamics of nematic spheroids

“…This geometry can be considered a rotational extrusion of a two-dimensional ellipse about its minor axis. It is therefore comparable to previous simulations of nematic elliptic capillaries, 20 in which a sequence of three different growth regimes were identified during droplet formation: (i) free growth, (ii) defect formation, and (iii) bulk relaxation. The free growth regime consists of the stable nematic "shell" growing into an unstable isotropic phase, with the bulk nematic orientation being commensurate with the homeotropic surface anchoring conditions.…”

“…This is due to a slowing down of the macroscale field-alignment in the bulk domain as the disclination loop diameter approaches a critical value where adjacent defect core regions interact. 20 Following this, the domain transitions to dynamic regime II-A which occurs rapidly followed by a long timescale regime II-B. For the oblate droplet case, the dynamic regime I is not observed to have two stages, implying different dynamics of the disclination loop during this regime.…”

“…The boundary is aligned perpendicular to the surface in accordance with the surface boundary conditions ref. 20.…”

“…10,11 While a significant body of past mesoscale simulation work exists for cylindrical nematic capillaries 12,13 and spherical droplets, [14][15][16] elliptic or ellipsoidal domains have been far less studied. [17][18][19][20] Furthermore, of this work, most use theoretical models which are unable to accurately capture nematic defects and phase transition. Only recently have simulations been performed which capture nematic dynamics, 21 as opposed to just determining equilibrium states.…”

Electrically induced structure transition in nematic liquid crystal droplets with conical boundary conditions

“…In this configuration the NLC droplet structure does exhibit an abrupt transition, when the electric field E exceeds some critical value E c , at which the point defect is transformed into the ring one forming the axial droplet structure ( Fig. 1, c) [30]. However, below this critical value, the director reorients along the field in the most part of the droplet, with simultaneous emergence of the wall perpendicular to the direction of applied voltage and crossing the point defect -hedgehog in the droplets centre.…”