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A study of substrate-liquid crystal ...

Zhang, Baoshe.

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A study of substrate-liquid crystal interaction.

Record Type:	Language materials, printed : Monograph/item
Title/Author:	A study of substrate-liquid crystal interaction./
Author:	Zhang, Baoshe.
Description:	186 p.
Notes:	Adviser: Ping Sheng.
Contained By:	Dissertation Abstracts International64-02B.
Subject:	Physics, Condensed Matter. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3081237

A study of substrate-liquid crystal interaction.
Zhang, Baoshe.

A study of substrate-liquid crystal interaction. - 186 p.

Adviser: Ping Sheng.

Thesis (Ph.D.)--Hong Kong University of Science and Technology (People's Republic of China), 2003.

This thesis concerns the study of substrate-liquid crystal interaction from two different angles. In one approach, we used the IPS (in-plane switching) technique to investigate the liquid crystal alignment by rubbed polyimide films. The IPS mode of liquid crystal cell operation is facilitated through comb electrodes capable of producing planar electric field. We have fabricated comb electrodes with a periodicity of 2 μ<italic>m</italic> in order to confine the planar electric field close to the liquid crystal-substrate interface. Through optical transmittance measurements and comparison with theoretical predictions based on the Ladau-de Gennes formalism, we found the experimental data to be consistent with the physical picture of soft anchoring, in which the liquid crystal director at the substrate interface is rotated azimuthally under the planar electric field. As a result, we were able to obtain the azimuthal anchoring strength as a fitting parameter of the theory. This part of the thesis thus presents evidence(s) for director switching at the liquid crystal-substrate interface, as well as a method for measuring the azimuthal anchoring strength through optical means. In the second approach, we used nano-lithographic technique to fabricate textured two dimensional periodic patterns on silicon wafers, and examined the resulting liquid crystal alignment effect of such textured substrates. It was found that with decreasing periodicity, there exists an orientational transition from a state in which the liquid crystal alignment copies the substrate pattern at larger periodicity, to a state of uniform alignment at smaller periodicity. In our system, this transition occurs at a periodicity between 0.4 μ<italic>m</italic> and 0.8 μ<italic>m</italic>. Through theoretical simulations based on the model of competition between the elastic distortion energy and the interfacial anchoring potential, it was found that there is indeed a first-order abrupt transition when the periodicity is decreased. This is due to the fact that the elastic distortion energy scales as the inverse of the periodicity squared. Hence when the periodicity is decreased, the elastic distortion energy increases rapidly. At the critical periodicity the elastic distortion energy crosses the interfacial anchoring potential, below which the uniform alignment becomes the lower energy state. The uniform-aligned state was confirmed by the excellent theory-experiment agreement on spectral measurements, in conjunction with the optical microscope observations. In the uniform-aligned state, a large pretilt angle (35°) was obtained.Subjects--Topical Terms:

1018743
Physics, Condensed Matter.

A study of substrate-liquid crystal interaction.
LDR:03528nam 2200277 a 45 001 934696
005 20110509
008 110509s2003 eng d
035 $a (UnM)AAI3081237
035 $a AAI3081237
040 $a UnM $c UnM
100 1 $a Zhang, Baoshe. $3 1258395
245 1 0 $a A study of substrate-liquid crystal interaction.
300 $a 186 p.
500 $a Adviser: Ping Sheng.
500 $a Source: Dissertation Abstracts International, Volume: 64-02, Section: B, page: 0778.
502 $a Thesis (Ph.D.)--Hong Kong University of Science and Technology (People's Republic of China), 2003.
520 $a This thesis concerns the study of substrate-liquid crystal interaction from two different angles. In one approach, we used the IPS (in-plane switching) technique to investigate the liquid crystal alignment by rubbed polyimide films. The IPS mode of liquid crystal cell operation is facilitated through comb electrodes capable of producing planar electric field. We have fabricated comb electrodes with a periodicity of 2 μ<italic>m</italic> in order to confine the planar electric field close to the liquid crystal-substrate interface. Through optical transmittance measurements and comparison with theoretical predictions based on the Ladau-de Gennes formalism, we found the experimental data to be consistent with the physical picture of soft anchoring, in which the liquid crystal director at the substrate interface is rotated azimuthally under the planar electric field. As a result, we were able to obtain the azimuthal anchoring strength as a fitting parameter of the theory. This part of the thesis thus presents evidence(s) for director switching at the liquid crystal-substrate interface, as well as a method for measuring the azimuthal anchoring strength through optical means. In the second approach, we used nano-lithographic technique to fabricate textured two dimensional periodic patterns on silicon wafers, and examined the resulting liquid crystal alignment effect of such textured substrates. It was found that with decreasing periodicity, there exists an orientational transition from a state in which the liquid crystal alignment copies the substrate pattern at larger periodicity, to a state of uniform alignment at smaller periodicity. In our system, this transition occurs at a periodicity between 0.4 μ<italic>m</italic> and 0.8 μ<italic>m</italic>. Through theoretical simulations based on the model of competition between the elastic distortion energy and the interfacial anchoring potential, it was found that there is indeed a first-order abrupt transition when the periodicity is decreased. This is due to the fact that the elastic distortion energy scales as the inverse of the periodicity squared. Hence when the periodicity is decreased, the elastic distortion energy increases rapidly. At the critical periodicity the elastic distortion energy crosses the interfacial anchoring potential, below which the uniform alignment becomes the lower energy state. The uniform-aligned state was confirmed by the excellent theory-experiment agreement on spectral measurements, in conjunction with the optical microscope observations. In the uniform-aligned state, a large pretilt angle (35°) was obtained.
590 $a School code: 1223.
650 4 $a Physics, Condensed Matter. $3 1018743
650 4 $a Physics, Fluid and Plasma. $3 1018402
650 4 $a Physics, Optics. $3 1018756
690 $a 0611
690 $a 0752
690 $a 0759
710 2 0 $a Hong Kong University of Science and Technology (People's Republic of China). $3 1249812
773 0 $t Dissertation Abstracts International $g 64-02B.
790 $a 1223
790 1 0 $a Sheng, Ping, $e advisor
791 $a Ph.D.
792 $a 2003
856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3081237