Microfluidic structures from nematic liquid crystals

doctoral thesis

Abstract

Nematicity is an exciting material property, which in complex fluids relies on the orientational ordering of passive or active (self-driven) building blocks. Here, I present the results of my research on microfluidic structures in passive and active nematic fluids that are characterised by geometrically distinct and often topologically protected profiles of the relevant material fields (such as the orientational field and the velocity field). The results are obtained through numerical and analytical efforts, with the main methodological approach being the mesoscopic continuum nematodynamic modelling based on the order paramter tensor which is solved by the hybrid lattice Boltzmann method. The used mesoscopic approach fully accounts for the backflow effects --- i.e. strong coupling between the orientational ordering and the material flow --- that underlies the explored structures. Structural properties of the flow and the orientational field are investigated and controlled through externally induced (electric, optic, or pressure) fields , topology-inducing confinement, or material activity. In junctions of microchannels, I characterize an effective interaction between topological defect structures in the orientational and in the velocity field of a flowing nematic. The topic is further explored in porous networks of cylindrical channels, where a variety of stationary structures and the transition processes between them is observed. In a single microchannel, flow field is used as a control mechanism for growth or annihilation of structural domains. A twist instability in nematic microchannels is shown, and a phase diagram is obtained, from which the expected nematic structure at a given speed in the channel and at a given elastic anisotropy can be identified. In nematic cells with patterned anchoring profiles, stability of Skyrmion-like structures is explored in view of elastic anisotropy and saddle-splay elasticity. Generation of flow patterns for confined nematics is proposed, based on using external electric or optic fields to continually deform the nematic structure and generate backflow. The process of structure formation during a temperature quench is examined. Lastly, nematics driven by active materials or inherent activity are studied. An analytical approximation of the flow field generated by a microswimmer in a nematic liquid crystal is provided. Structural properties of unconfined and confined three-dimensional (3D) active nematics are investigated and a variety of regimes is identified. Main modes of coupled oscillations of active and passive defects in emulsions of active nematic droplets in the passive nematic medium are explored. The thesis is aimed towards control of the emergent properties and functionality of non-equilibrium soft matter through the manipulation of the underlying structural organization.

Keywords

nematic liquid crystals;nematodynamics;microfluidics;field structures;flow generation;microswimmers;active nematics;topological defects;

Data

Language:	English
Year of publishing:	2019
Typology:	2.08 - Doctoral Dissertation
Organization:	UL FMF - Faculty of Mathematics and Physics
Publisher:	[Ž. Kos]
UDC:	538.9(043.3)
COBISS:	3290980
Views:	1085
Downloads:	553
Average score:	0 (0 votes)
Metadata:

Other data

Secondary language:	Slovenian
Secondary title:	Mikrofluidne strukture na osnovi nematskih tekočih kristalov
Secondary abstract:	Nematska ureditev je lastnost nekaterih kompleksnih tekočin, ki se izrazi z orientacijskim redom pasivnih ali aktivnih gradnikov. V doktorski disertaciji predstavim rezultate svojih znanstvenih raziskav mikrofluidnih struktur pasivnih in aktivnih nematskih tekočin, za katere so značilni specifični geometrijski in pogosto tudi topološko zaščiteni profili relevantnih materialnih polj --- na primer orientacijskega polja ter hitrostnega polja. Rezultati so numerični in analitični, pri čemer je bil osrednji metodološki pristop mezoskopsko kontinuumsko modeliranje nematodinamike na osnovi tenzorskega ureditvenega parametra ter hibridne mrežne Boltzmannove metode. Tak pristop polno upošteva sklopitev med orientacijskim redom ter materialnim tokom, ki močno vpliva na raziskovane strukture. Strukturne lastnosti toka ter orientacijske ureditve so kontrolirane preko zunanjih polj (na primer električnega polja, optičnih žarkov, tlačnega polja), ograditve ter aktivnosti materiala. V stikih mikrokanalov sem pokazal lastnosti efektivne interakcije med topološkimi defekti v orientacijskem ter hitrostnem polju tekočega nematika. Problem sem nadalje raziskoval v poroznih omrežjih cilindričnih kanalov, kjer sem pokazal raznovrstne stacionarne strukture ter procese dinamičnih prehodov med temi strukturami. V enem samem mikrokanalu sem teoretično opisal uporabo hitrostnega polja za kontrolo rasti ali anihilacije strukturnih domen. Pokazal sem tudi obstoj zvojne nestabilnosti v nematskih mikrokanalih. V nematskih celicah z vzorci sidranja sem preučeval stabilnost skyrmionskih struktur glede na elastične lastnosti nematika. V disertaciji predlagam način gnanja tokovnih profilov v ograjenih nematskih geometrijah, ki je osnovan na zunanjih električnih in optičnih poljih, ki preko deformacije orientacijskega reda ženejo materialni tok. V doktoratu sem preučeval tudi proces formacije strukture med nenadnim faznim prehodom nematskega tekočega kristala. Zadnji predmet raziskovanja pa so nematske tekočine, gnane preko stika z aktivnimi materiali ali preko lastne aktivnosti. Primer takšne sklopitve je izpeljava analitičnega približka za hitrostno polje nematika okoli mikroplavalca. Preučeval sem strukturne lastnosti ograjenih in neograjenih tridimenzionalnih (3D) aktivnih nematikov v različnih režimih aktivnosti. V emulziji aktivnih nematskih kapljic v pasivnem nematiku sem identificiral osnovne načine sklopljene dinamike defektov. Če povzamem: osrednji cilj disertacije sta bila razumevanje in razvoj novih lastnosti ter funkcionalnosti neravnovesnih struktur v mehki snovi preko vpliva na elementarno strukturno organiziranost.
Secondary keywords:	nematski tekoči kristali;nematodinamika;mikrofluidika;strukture polja;generacija toka;mikroplavalci;aktivni nematiki;topološki defekti;
Type (COBISS):	Doctoral dissertation
Study programme:	0
Thesis comment:	Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages:	113, XII str.
ID:	11026492