Topological structures in active nematics

Kralj, Nika (avtor), Ravnik, Miha (mentor), Kos, Žiga (komentor)

Nematic order and collective dynamics characterize various biological and synthetic systems. In my dissertation, I study the dynamics of three dimensional active nematic systems. The main methodological approach is the mesoscopic continuum model of nematodynamics, based on the coupling of orientational order and flow field. Coupled equations are numerically solved using the hybrid lattice Boltzmann approach. Under the influence of the varying activity, topological defects are characterized by coarsening and refinement dynamics, where the dynamics of the entire system are modified by the evolution of individual defect loops. I develop a numerical model for the expansion and shrinkage of a defect loop and extend it to the dynamics in a three-dimensional system. I explore the influence of the activity on the coarsening and refinement of defect lines between dynamic regimes and present possible parallels with systems beyond active matter. I analyze how selected material parameters influence the dynamic steady state of the active nematic turbulence. I focus on the alignment parameter in shear flow and consider contractile and extensile systems separately. I study the influence of elastic anisotropy as caused by different contributions of the elastic constants. I compare characteristics of the achiral and chiral active turbulence and investigate the emergence of a novel active blue phase. I discuss the coupling of passive and active phase in nematic emulsions with emphasis on the dynamics of the hyperbolic hedgehog dipole and the Saturn ring dipole in case of various activities. I study the coherence of disclination loop oscillations located around the coupled dimer structures. Finally, this work is a contribution towards understanding of new active matter structures, especially those conditioned by topological defects in three spatial dimensions.

• Jezik: Angleški jezik
• Ključne besede: active matter, active nematics, topological defects, active turbulence, liquid crystals, active nematic emulsions
• Vrsta gradiva: Doktorsko delo/naloga
• Tipologija: 2.08 - Doktorska disertacija
• Organizacija: FMF - Fakulteta za matematiko in fiziko
• Leto izida: 2025
• PID: 20.500.12556/RUL-175423
• COBISS.SI-ID: 254983939
• Datum objave v RUL: 26.10.2025

Sekundarni jezik

• Jezik: Slovenski jezik

Izvleček

Nematska urejenost in aktivna kolektivna dinamika sta značilni za različne biološke in sintetične sisteme. V disertaciji preučujem dinamiko aktivnih nematskih struktur v treh dimenzijah. Osrednji uporabljen metodološki pristop je mezoskopsko kontinuumsko modeliranje nematodinamike, ki temelji na sklopitvi orientacijskega reda in materialnega toka ter se rešuje z uporabo hibridne mrežne Boltzmannove metode. Pod vplivom spreminjanja velikosti aktivnosti se topološke defektne linije redčijo in zgoščujejo, spremembe celotnega sistema pa izhajajo iz dinamike posameznih defektnih zank. Razvijem numerični pristop za širjenje in krčenje defektne zanke in ga razširim na dinamiko defektnih linij v tridimenzionalnem sistemu. Pokažem vpliv aktivnosti na razvoj defektnih linij med dinamičnimi režimi in predstavim možne vzporednice s sistemi preko aktivne snovi. Analiziram vpliv materialnih parametrov na izračun dinamičnega ravnovesja v aktivni nematski turbulenci. Osredotočim se na parameter urejenosti molekul v strižnem toku, pri čemer ločeno obravnavam molekule v kontraktilnih in ekstenzilnih sistemih. Preučim vpliv elastične anizotropije z ločeno analizo posameznih prispevkov elastičnih konstant. Primerjam dinamične značilnosti akiralne in kiralne aktivne turbulence ter raziščem pojav nove aktivne modre faze. Nazadnje obravnavam sklopitev pasivne in aktivne faze v nematskih emulzijah ter analiziram dinamiko v kvadrupolnih in dipolnih nematskih strukturah pri različnih stopnjah aktivnosti. Analiziram koherenco oscilacij defektne zanke v okolici sklopljenih dimernih struktur. Delo je prispevek k razumevanju novih aktivnih mehkih snovi, posebej tistih pogojenih s topološkimi defekti v treh prostorskih dimenzijah.

• Ključne besede: aktivna snov, aktivni nematiki, topološki defekti, aktivna turbulenca, tekoči kristali, aktivne nematske emulzije