Ferroelectric smectic-A phase made of bent-core liquid crystals: Structure and Dielectric response in thin cells

Leskovar, Kristina

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Title:	Ferroelectric smectic-A phase made of bent-core liquid crystals: Structure and Dielectric response in thin cells
Authors:	ID Leskovar, Kristina (Author) ID Vaupotič, Nataša (Mentor) More about this mentor...
Files:	DOK_Leskovar_Kristina_2016.pdf (4,05 MB) MD5: C395F8D6CC578E78BF331AA258E7545D
Language:	English
Work type:	Doctoral dissertation
Typology:	2.08 - Doctoral Dissertation
Organization:	FNM - Faculty of Natural Sciences and Mathematics
Abstract:	In the thesis we study the structure and response of bent-core liquid crystals in the orthogonal ferroelectric Smectic-A (SmAP $_F$ ) phase in thin planar cells. We construct a phenomenological continuum model to study the structure in thin planar cells. A set of molecules within a small volume is presented by the director ( $vec{n}$ ), which defines the average direction of the long molecular axes within this volume, and by the polar director ( $vec{p}$ ), which points in the direction of local polarization. We choose a geometry in which the director in the smectic layer is constant and the polar director varies across the cell. The polar director structure inside the cell is determined by a competition among the torques due to the bulk elasticity, electrostatic effects and surface anchoring. The equilibrium profile structure of the polar director is obtained by minimization of the free energy. We find the polar director profile in a cell as a function of the type and strength of the surface anchoring, bulk elastic constants and cell thickness. The effect of the external electric bias field on the structure in the cell is studied, as well. Bent-core molecules have a permanent electric dipole moment. In the external electric field the electric torque tends to rotate the dipoles in the direction of the external field. The polar director profile in the cell in external bias field thus depends on the competition among three effects: anchoring at the surfaces, the elastic properties of the bulk and the influence of the external electric field. By the rotation of molecules in the external electric field, optical properties of the cell are changed, therefore the SmAP $_F$ phase is a promising phase for use in displays with high response time, high contrast, continuous gray level and wide viewing angle. The response of the SmAP $_F$ phase to an alternating external electric field (the dielectric response) is also considered. The dielectric response of the SmAP $_F$ phase consists of two modes: the phase and amplitude mode. The phase mode is due to fluctuations in the orientation of the local direction of the spontaneous polarization and the amplitude mode is due to the change in the magnitude of spontaneous polarization. The frequency of the phase and amplitude mode and the dielectric permittivity are calculated numerically as a function of the bias external DC electric field, cell thickness, the type and strength of surface anchoring and the ratio between the bend and splay elastic constants. Analytical solution for a very specific case of chosen parameters is also obtained. Theoretically obtained dependencies are in agreement with the reported experimental measurements. In the thesis we study the effect of different types of surface anchoring and for this purpose three different types of cells are defined. The polar director profile and the dielectric response is calculated in all three types of cells. The type I cell has polar surface anchoring of equal strengths at both surfaces. The type II cell has, in addition to the polar surface anchoring of equal strengths at both surfaces, a nonpolar anchoring at the bottom surface. The type III cell has polar surface anchoring at both surfaces but not of equal strengths. We predict that by comparison of the dielectric response of the SmAP $_F$ phase in all three types of cells the type of the polarization splay in bent-core liquid crystals can be determined. The thesis is divided into three parts. In the first part the basic physical properties of ferroelectric liquid crystals, focusing on the bent-core liquid crystals, are discussed. In the second part phenomenological theoretical model is developed. In the third part of the thesis a dielectric response in external bias field is studied.
Keywords:	Bent-core liquid crystals, ferroelectric smectic-A phase, dielectric response, confined geometry, phenomenological continuum model, surface anchoring, polarization splay
Place of publishing:	[Maribor
Publisher:	K. Leskovar]
Year of publishing:	2016
PID:	20.500.12556/DKUM-57326
UDC:	532.783(043.3)
COBISS.SI-ID:	22166792
NUK URN:	URN:SI:UM:DK:IUVRGLYV
Publication date in DKUM:	06.05.2016
Views:	1989
Downloads:	166
Metadata:
Categories:	FNM
:	LESKOVAR, Kristina, 2016, Ferroelectric smectic-A phase made of bent-core liquid crystals: Structure and Dielectric response in thin cells [online]. Doctoral dissertation. Maribor : K. Leskovar. [Accessed 27 April 2025]. Retrieved from: https://dk.um.si/IzpisGradiva.php?lang=eng&id=57326 Copy citation

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Secondary language

Language:	Slovenian
Title:	Feroelektrična smektična-A tekočekristalna faza iz molekul z ukrivljeno sredico: struktura in dielektrični odziv v tankih celicah
Abstract:	V doktorski disertaciji obravnavamo strukturo in dielektrični odziv tekočih kristalov z ukrivljeno sredico v feroelektrični smetični-A (SmAP $_F$ ) fazi v tankih planarnih celicah. Za preučevanje struktur v tankih planarnih celicah zgradimo minimalni fenomenološki kontinuumski model. Skupek molekul v majhni prostornini opišemo z direktorjem ( $vec{n}$ ), ki določa povprečno usmerjenost vseh dolgih osi molekul v skupku, in s polarnim direktorjem ( $vec{p}$ ), ki kaže v smeri lokalne polarizacije. V izbrani geometriji je direktor v smektičnih plasteh konstanten, spreminja se le smer polarnega direktorja. Ravnovesno strukturo polarnega direktorja v celici določa tekmovanje med navori zaradi elastičnosti, elektrostatičnega učinkom in površinskega sidranja. Dobimo jo z minimizacijo proste energije. Strukturo polarnega direktorja v celici poiščemo v odvisnosti od vrste in jakosti površinskega sidranja, velikosti elastičnih konstant ter od debeline celice. Obravnavamo tudi strukturo krajevnega ureditvenega parametra v celici v odvisnosti od jakosti konstantnega zunanjega električnega polja. Ukrivljene molekule tekočih kristalov imajo permanentni električni dipol. V zunanjem električnem polju na dipol deluje navor, ki želi zavrteti dipol v smeri polja. V tem primeru je krajevna odvisnost polarnega direktorja odvisna od površinskega sidranja, elastičnih lastnosti materiala in jakosti polja. Z vrtenjem dipolov v zunanjem električnem polju se spremenijo optične lastnosti tekočekristalne celice, zato je SmAP $_F$ faza obetavna za uporabo v zaslonih s hitrim odzivnim časom, visoko ločljivostjo, dobrim kontrastom, zvezno sivo skalo ter širokim vidnim kotom. Še posebej nas zanima dielektrični odziv SmAP $_F$ faze. Z izmeničnim zunanjim poljem na tekoči kristal delujemo na dva načina: zanihamo smer lokalne polarizacije, torej smer polarnega direktorja (fazni način nihanja), in lokalno večamo in manjšamo stopnjo urojenosti električnih dipolov ter s tem velikost lokalne polarizacije (amplitudni način nihanja). Z numeričnim pristopom raziščemo, kako jakost električnega polja, debelina celice, jakost ter vrsta površinskega sidranja in razmerje velikosti med elastičnima konstantama za upogibno in pahljačasto deformacijo, vplivajo na frekvenco faznega in amplitudnega načina nihanja. Za specifično izbrane parametre izračunamo tudi analitično rešitev. Napovedi teoretičnega modela se ujemajo z rezultati obstoječih eksperimentalnih meritev. V disertaciji preučimo tudi učinek različnih vrst sidranj na površini in zato definiramo tri različne vrste celic. V vseh treh vrstah poiščemo ravnovesno strukturo polarnega direktorja ter dielektrični odziv. V prvi vrsti celice je na obeh površinah celice enako močno polarno sidranje. V drugi vrsti dodamo poleg enako močnega polarnega sidranja na obeh površinah še nepolarno sidranje na spodnji površini. V tretji vrsti celice je na obeh površinah polarno sidranje, ki pa ni enako močno. Napovemo, da s primerjavo dielektričnega odziva SmAP $_F$ faze v vseh treh vrstah celic lahko določimo vrsto pahljačaste deformacije spontane polarizacije v tekočih kristalih iz ukrivljenih molekul. Doktorsko delo je sestavljeno iz treh sklopov. V prvem predstavimo osnovne fizikalne lastnosti feroelektričnih tekočih kristalov s poudarkom na lastnostih tekočih kristalov iz ukrivljenih molekul. V drugem sklopu razvijemo fenomenološki kontinuumski model, v tretjem delu pa obravnavamo dielektrični odziv v zunanjem električnem polju.
Keywords:	Tekoči kristali z ukrivljeno sredico, feroelektrična smektična-A faza, dielektrični odziv, omejena geometrija, fenomenološki kontinuumski model, površinsko sidranje, pahljačasta deformacija spontane polarizacije

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