Sekundarni jezik: |
Angleški jezik |
Sekundarni naslov: |
Liquid crystals formed by bent-core molecules |
Sekundarni povzetek: |
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? |
Sekundarne ključne besede: |
physics;fizika; |
Vrsta datoteke: |
application/pdf |
Vrsta dela (COBISS): |
Diplomsko delo |
Strani: |
III, 26 f. |
Vrsta dela (ePrints): |
thesis |
Naslov (ePrints): |
Liquid crystals formed by bent-core molecules |
Ključne besede (ePrints): |
tekoči kristali |
Ključne besede (ePrints, sekundarni jezik): |
liquid crystals |
Povzetek (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? |
Povzetek (ePrints, sekundarni jezik): |
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? |
Ključne besede (ePrints, sekundarni jezik): |
liquid crystals |
ID: |
8310166 |