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Structural disjoining pressure in thin film of liquid crystals I.: Thermodynamics and Frederiksz transition with surface fields

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  • Colloid Science
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Summary

Thin films (0,2 – 10μm) of a nematic liquid crystal NLC: 4-4′-pentylcyanobiphenyl (5CB) have been spread on large surfaces of water.

The orientation of the molecules has been examined between crossed polarizer und analyzer und by studying the average retardation of white light as a function of film thickness. Ath T ≌ 2.2μm a Frederiksz transition takes place owing to the balance of elastic and surface forces acting on the film and determining the molecular orientations in bulk at the surface as a function of films thickness. From this variation andh T , an anisotropy of surface tension of 5CB of the order of 10−6 Jm−2 is found. This weak anisotropy determines also the variation of the structural disjoining pressure und the film stability which is discussed at length. A thermodynamic potential for this films is suggested.

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Abbreviations

h T :

film transition thickness

h s :

critical film stability thickness

ψ superscript:

film phase

Λ :

area

f :

force per unit area

h :

film thickness

T :

temperature

V :

volume

F :

free energy

η :

disjoining pressure

d τ :

work performed in varyingh

P c :

capillary pressure

s :

specific surface entropy

Δ :

film tension

μ :

chemical potential

σ :

surface tension

Λ(h) :

number of molecules per unit surface of film at thicknessh

\(\bar \omega \) :

film perimeter

subscripth :

upper surface

subscripto :

lower surface

w :

surface free enthalpy

G :

free enthalpy

W′:

specific bulk free enthalpy in film of L. C.

W b :

specific bulk free enthalpy of L.C.

W(h,z) :

specific excess free enthalpy in thin film of L.C.

\(\mathop n\limits^ \to \) :

liquid crystal director

0 :

angle between director andx, y plan

ϕ :

azimutal angle

W ′ d :

elastic energy density

W ′ u :

specific bulk free enthalpy of the undistorted film

w′,Δw′:

isotropic and anisotropic contribution to the interfacial tension of L.C.

K :

average elastic constant of nematic liquid crystal

Δ u :

non elastic contribution to the film tension

Δ d :

elastic contribution to the film tension

Λ el :

elastic torque

Λ s :

surface torque

η d :

classical disjoining pressure

η s :

structural disjoining pressure

B(h) :

Van der Waals parameter

Δ n :

optical anisotropy of the liquid crystal

δ :

optical retardation

I :

light intensity

h + :

reduced thickness

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Authors and Affiliations

  1. Equipe de Recherche C. N. R. S. associée à l'Université Paris V, U.E.R. Biomédicales 81, Paris, France

    E. Perez, J. E. Proust &  L. Ter-Minassian-Saraga

Authors
  1. E. Perez
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  2. J. E. Proust
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  3. L. Ter-Minassian-Saraga
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Additional information

With 8 figures

Part II of this paper has been published in the issue: No 255, 1133–1135 of this journal.

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Perez, E., Proust, J.E. & Ter-Minassian-Saraga, L. Structural disjoining pressure in thin film of liquid crystals I.: Thermodynamics and Frederiksz transition with surface fields. Colloid & Polymer Sci 256, 784–792 (1978). https://doi.org/10.1007/BF01438035

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  • DOI: https://doi.org/10.1007/BF01438035

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