Field structures in active and passive liquid crystals

Aplinc, Jure (author), Ravnik, Miha (mentor)

Field structures are developed in passive and active nematic fluids. These are field profiles that are determined by confinement, particles, flow and external fields. Our central methodological approach is numerical modeling based on free energy minimization with finite difference method and flow modeling with hybrid lattice Boltzmann method. We develop structures by combining concepts of topological defects, external confinement and colloidal particles. Ordering properties of horseshoe nematic colloidal particles with planar degenerate anchoring are investigated with numerical modeling, where we optimize their geometrical parameters such that the particle exhibit attractive interactions and can self assemble into 2D and even 3D colloidal crystals. The metamaterial response of horseshoe colloids that perform as split ring resonators is studied. Optical cloaking is achieved by generating polymer microstructures embedded directly within a electric field switchable liquid crystal device. Using numerical modelling we explore the director field structures forming in the vicinity of composite colloidal particles with specially designed conic anchoring, which are assumed to induce high multipoles. Simple rule that allow predictions of multipolar moment from defect configuration is extracted. Starting with a gyroid structure, which is a photonic crystal by itself, we introduce an achiral and chiral nematic into one labyrinth of channels with homeotropic anchoring. Complexly shaped channels induce both ordered and disordered structures of defects. Simulating the passive nematic flow in porous microchannels we study the formation of individual umbilic defects of various strength and umbilic defect lattices that arise as the consequence of complex velocity field containing both multiple peaks and saddles. We investigate the 3D active turbulence in droplets of active nematic with homeotropic and non slip boundary condition. The transition from the point defect to the active turbulence is studied by analysing both the topological defects and corresponding events as well as flow. More generally, this work is aimed at the development of novel functional soft matter, which can exhibit exciting and unusual material characteristics, including light guiding, topological defect states, photonic bandgaps, metamaterials and optical cloaking.

• Language: English
• Keywords: field structures, topological defects, tunability, nematic liquid crystals, colloids, optical cloaking, metamaterials, nematofluidics, active nematics
• Work type: Doctoral dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2018
• PID: 20.500.12556/RUL-101532
• COBISS.SI-ID: 3207524
• Publication date in RUL: 14.06.2018

Secondary language

• Language: Slovenian
• Title: Strukture polja v aktivnih in pasivnih tekočih kristalih

Abstract

V doktorskem delu smo razvili strukture polja v pasivnih in aktivnih nematskih tekočinah. Ti profili v polju so določeni z ograditvijo, delci, tokom in zunanjimi polji. Osrednji raziskovalni pristop je numerično modeliranje, ki temelji na minimizaciji proste energije z metodo končnih diferenc, in modeliranje toka s hibridno mrežno Boltzmannovo metodo. Ustvarjene strukture so rezultat kombinacije topoloških defektov, zunanje ograditve in koloidnih delcev. Preučevali smo urejanje podkvastih koloidnih delcev s planarnim sidranjem. Geometrijske parametre koloidnega delca smo optimizirali tako, da so delci medsebojno interagirali privlačno in so se lahko sestavili v 2D in tudi 3D koloidne kristale. Študirali smo tudi metamaterialni odziv tovrstnih podkvastih koloidov, ki se obnašajo kot resonatorji. Pokazali smo optično zakrivanje z ustvarjanjem polimernih struktur direktno v tekočekristalni celici, nastavljivi z električnim poljem. S pomočjo numeričnega modeliranja smo raziskali strukture v nematskem polju, ki se formirajo v okolici kompozitnih koloidnih delcev s posebnim koničnim sidranjem in ustvarjajo višje multipolne momente. Predstavimo tudi preprosto pravilo, s katerim lahko napovemo multipolni moment samo z opazovanjem defektnih struktur. V enega od obeh prepletov kanalov, v giroidni strukturi, uvedemo kiralni in nekiralni nematski tekoči kristal. Kompleksna oblika kanalov povzroči nastanek tako urejenih, kot tudi neurejenih defektnih struktur. Simuliramo pasivni nematski tok v poroznih mikrokanalih in študiramo nastanek umbiličnih defektov različnih moči ter regularnih mrež umbiličnih defektov, ki nastanejo zaradi sedelnih in ekstremalnih točk v toku. Preučimo 3D aktivno turbulenco v kapljicah aktivnega nematika s homeotropnimi robnimi pogoji. Študiramo prehod iz točkastega defekta v topološko turbulenco z analizo topoloških defektov in topoloških dogodkov, kot tudi z analizo samega toka. To delo je torej namenjeno razvoju nove funkcionalne mehke snovi, ki ima zanimive lastnosti, kot so na primer vodenje svetlobe, topološka defektna stanja, fotonske reže, metamateriali in optično zakrivanje.

• Keywords: strukture polja, topološki defekti, nastavljivost, nematski tekoči kristal, koloidi, optično zakrivanje, metamateriali, nematofluidika, aktivni nematiki