Tekoči kristali z ukrivljeno sredico

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Language:	Slovenian
Year of publishing:	2011
Source:	Ljubljana
Typology:	2.11 - Undergraduate Thesis
Organization:	UL PEF - Faculty of Education
Publisher:	[K. Gornik]
UDC:	532.783(043.2)
COBISS:	9231177
Views:	1002
Downloads:	225
Average score:	0 (0 votes)
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Secondary language:	English
Secondary title:	Liquid crystals formed by bent-core molecules
Secondary abstract:	Liquid crystals formed by bent-core molecules are studied in the thesis. Bent-core liquid crystals are extremely interesting for technological applications because weak external electric field has major impact on their optical properties. However, bent-core liquid crystals are interesting also because of their basic physical properties: they form polar liquids and most surprisingly, achiral molecules form chiral structures. In the thesis we study the alignment of bent-core liquid crystal in cells. The structure inside the cell is affected by surfaces. If anchoring at the surface is strong, the orientation of molecules at the surface is defined. Since the elastic properties of the interior of liquid crystal (bulk) in general prefer different alignment than the surface, the alignment of molecules in the cell depends on the cell thickness. Bent-core molecules have a permanent electric dipole moment. In the external electric field electrical torque wants to rotate the dipole in the direction of external field. The orientation of molecules in the cell in external 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 rotation of molecules in the external electric field the optical properties of the cell are changed, e.g. when the field is switched of the cell transmits light and with the field switched on it does not transmit light. This is the reason, why liquid crystals are so widely used in displays. The thesis is divided into three parts. In the first part the basic physical properties of liquid crystals, especially bent-core liquid crystals, are discussed. In the second part phenomenological theoretical model is introduced. A set of molecules within a small volume is presented by the director, which defines the direction of the long molecular axes within this volume, and by the polar director, which points in the direction of local polarization. We write down the free energy of the system at given conditions and minimize it to find the equilibrium spatial dependence of polar director as a function of the cell thickness and the intensity of the external electric field. In the third part a construction of a mechanical teaching device is presented. With the teaching device it is possible to show the arrangement of bent-core liquid crystal molecules in bulk, near surfaces and in cells with and without the external field. The thesis gives answers to the following questions. What is the alignment of bent-core molecules in cells of different thicknesses? How do bent-core liquid crystals respond to an external electric field applied to the cell? How can one present the liquid crystal structure and its response in external field to pupils?
Secondary keywords:	physics;fizika;
File type:	application/pdf
Type (COBISS):	Undergraduate thesis
Pages:	III, 26 f.
Type (ePrints):	thesis
Title (ePrints):	Liquid crystals formed by bent-core molecules
Keywords (ePrints):	tekoči kristali
Keywords (ePrints, secondary language):	liquid crystals
Abstract (ePrints):	V diplomskem delu obravnavamo tekoče kristale, ki jih tvorijo molekule z ukrivljeno sredico. Tovrstni tekoči kristali so izjemno zanimivi za tehnološko uporabo, ker z zunanjim elektri£nim poljem šibkih jakosti močno vplivamo na optične lastnosti. Predvsem pa so zanimivi s stališ£a preučevanja temeljnih fizikalnih lastnosti, saj molekule tvorijo polarne tekočine. Poleg tega zrcalno simetrične molekule tvorijo zrcalno nesimetrične strukture. V diplomskem delu obravnavamo urejanje tekočih kristalov iz ukrivljenih molekul v tekočekristalnih celicah. Na urejanje vplivata površini, ki določata točno določeno ureditev molekul tik ob površini in elastične lastnosti notranjosti tekočega kristala, ki v splošnem želijo drugačno ureditev kot površina. Ureditev molekul v celici je zato odvisna od razdalje med površinama. Ukrivljene molekule imajo stalni električni dipolni moment, zato nanje v zunanjem električnem polju deluje električni navor, ki poskuša molekule zavrteti z električnim dipolom v smeri zunanjega polja. Končna ureditev v celici je odvisna od tekmovanja med tremi učinki: vplivov površine, elastičnih lastnosti tekočega kristala in vplivov zunanjega električnega polja. Ko se molekule v zunanjem polju zavrtijo, se spremenijo optične lastnosti tekočekristalne celice. Ko polja ni, celica prepušča svetlobo, ko vklopimo polje, celica svetlobe ne prepušča. Zato so tekoči kristali tako zelo razširjeni prav v številnih prikazovalnikih. Diplomsko delo je sestavljeno iz treh delov. V prvem delu predstavimo osnovne fizikalne lastnosti tekočih kristalov. Posebej se posvetimo tekočim kristalom z ukrivljeno sredico. Drugi del zajema fenomenološki teoretični model. Skupek molekul predstavimo z direktorjem, ki kaže v smeri ureditve dolgih osi molekul, in s polarnim direktorjem, ki kaže v smeri lokalne polarizacije skupka molekul. Zapišemo prosto energijo sistema ob danih pogojih na površini in poišćemo ravnovesno krajevno odvisnost smeri polarnega direktorja v odvisnosti od debeline celice in jakosti zunanjega električnega polja. V tretjem delu predstavimo konstrukcijo mehanskega učila. Z njim učencem predstavimo urejanje ukrivljenih molekul tekočega kristala. Na ureditev vplivata površini in zunanje električno polje. V diplomskem delu odgovorimo na sledeča vprašanja. Kako se urejajo ukrivljene molekule tekočega kristala v odvisnosti od debeline celice? Kakšen je odziv ukrivljenih molekul v tekočekristalni celici, če jo postavimo v zunanje električno polje? Kako odziv molekul v tekočem kristalu predstavimo učencem v osnovni šoli?
Abstract (ePrints, secondary language):	Liquid crystals formed by bent-core molecules are studied in the thesis. Bent-core liquid crystals are extremely interesting for technological applications because weak external electric field has major impact on their optical properties. However, bent-core liquid crystals are interesting also because of their basic physical properties: they form polar liquids and most surprisingly, achiral molecules form chiral structures. In the thesis we study the alignment of bent-core liquid crystal in cells. The structure inside the cell is affected by surfaces. If anchoring at the surface is strong, the orientation of molecules at the surface is defined. Since the elastic properties of the interior of liquid crystal (bulk) in general prefer different alignment than the surface, the alignment of molecules in the cell depends on the cell thickness. Bent-core molecules have a permanent electric dipole moment. In the external electric field electrical torque wants to rotate the dipole in the direction of external field. The orientation of molecules in the cell in external 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 rotation of molecules in the external electric field the optical properties of the cell are changed, e.g. when the field is switched of the cell transmits light and with the field switched on it does not transmit light. This is the reason, why liquid crystals are so widely used in displays. The thesis is divided into three parts. In the first part the basic physical properties of liquid crystals, especially bent-core liquid crystals, are discussed. In the second part phenomenological theoretical model is introduced. A set of molecules within a small volume is presented by the director, which defines the direction of the long molecular axes within this volume, and by the polar director, which points in the direction of local polarization. We write down the free energy of the system at given conditions and minimize it to find the equilibrium spatial dependence of polar director as a function of the cell thickness and the intensity of the external electric field. In the third part a construction of a mechanical teaching device is presented. With the teaching device it is possible to show the arrangement of bent-core liquid crystal molecules in bulk, near surfaces and in cells with and without the external field. The thesis gives answers to the following questions. What is the alignment of bent-core molecules in cells of different thicknesses? How do bent-core liquid crystals respond to an external electric field applied to the cell? How can one present the liquid crystal structure and its response in external field to pupils?
Keywords (ePrints, secondary language):	liquid crystals
ID:	8310166