Forces due to structure in a thin liquid crystal film

Horn, R. G., Israelachvili, J. N. and Perez, E. 1981, Forces due to structure in a thin liquid crystal film, Journal de Physique, vol. 42, no. 1, pp. 39-52, doi: 10.1051/jphys:0198100420103900.

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Title Forces due to structure in a thin liquid crystal film
Author(s) Horn, R. G.
Israelachvili, J. N.
Perez, E.
Journal name Journal de Physique
Volume number 42
Issue number 1
Start page 39
End page 52
Total pages 14
Publisher Springer
Place of publication Amsterdam, The Netherlands
Publication date 1981-01
ISSN 1434-6028
Keyword(s) elastic deformation
liquid films
nematic liquid crystals
refractive index
Summary Measurements of the force as a function of distance between two solids separated by a liquid crystal film give information on the structure of the film. We report such measurements for two molecularly smooth surfaces of mica separated by the nematic liquid crystal 4'-n-pentyl 4-cyanobiphenyl (5CB) in both the planar and homeotropic orientations at room temperature. The force is determined by measuring the deflection of a spring supporting one of the mica pieces, while an optical technique is used to measure the film thickness to an accuracy of ± (0.1-0.2) nm. The technique also allows the refractive indices of the nematic to be measured, and hence a determination of the average density and order parameter of the liquid crystal film as a function of its thickness. Three distinct forces were measured, each reflecting a type of ordering of the liquid crystal near the mica surfaces. The first one results from elastic déformation in the liquid crystal ; it was only observed in a twisted planar sample where the 5CB molecules are oriented in different directions at the two mica surfaces. The second, measured in both the planar and homeotropic orientations, is attributed to an enhanced order parameter near the surfaces. Both of these are monotonic repulsive forces measurable below 80 nm. Finally, there is a short-range force which oscillates as a function of thickness, up to about six molecular layers, between attraction and repulsion. This results from ordering of the molecules in layers adjacent to the smooth solid surface. It is observed in both the planar and homeotropic orientations, and also in isotropic liquids.
Language eng
DOI 10.1051/jphys:0198100420103900
Field of Research 029999 Physical Sciences not elsewhere classified
Socio Economic Objective 970102 Expanding Knowledge in the Physical Sciences
HERDC Research category C1.1 Refereed article in a scholarly journal
Copyright notice ©1981, Springer
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