Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence (Delta n) of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to 2.3 microm) and thick (7 to 16 microm) cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in...

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Bibliographic Details
Published in:The European physical journal. E, Soft matter and biological physics Soft matter and biological physics, 2004-04, Vol.13 (4), p.401-408
Main Authors: DHARA, Surajit, MADHUSUDANA, N. V
Format: Article
Language:English
Subjects:
Biophysics - methods
Condensed matter: structure, mechanical and thermal properties
Exact sciences and technology
Glass
Imides - chemistry
Liquid crystals
Models, Chemical
Models, Statistical
Physics
Polymers - chemistry
Structure of solids and liquids
crystallography
Temperature
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Summary:We report measurements of birefringence (Delta n) of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to 2.3 microm) and thick (7 to 16 microm) cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to 18%) of Delta n (proportional to S, where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far approximately 20 degrees C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.
ISSN:1292-8941
1292-895X
DOI:10.1140/epje/i2003-10084-8