Surface tension of freestanding partially fluorinated liquid-crystal films

Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics

Volumn 53, Issue 2, 1996, Pages 1662-1665

  Stoebe, T ^a,b   Mach, P ^a,b   Grantz, S ^a,b   Huang, C C ^a,b  

^a UNIVERSITY OF MINNESOTA   (United States)

^b University of Minnesota   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000259251     PISSN: 1063651X     EISSN: None     Source Type: Journal    
DOI: 10.1103/PhysRevE.53.1662     Document Type: Article

References (24)

1. T. Stoebe, P. Mach and C. C. Huang, Phys. Rev. E 49, R3587 (1994).
2. E. P. Janulis, D. W. Osten, M. D. Radcliffe, J. C. Novack, M. Tristani-Kendra, K. A. Epstein, M. Keyes, G. C. Johnson, P. M. Savu and T. D. Spawn, SPIE Int. Soc. Opt. Eng. 166E, 143 (1992).
3. Particularly in the case of ferroelectric devices pioneered by N. A. Clark and S. T. Lagerwell, Appl. Phys. Lett. 36, 899 (1980).
4. G. W. Gray and J. W. Goodby, Smectic Liquid Crystals (Leonard Hill, Glasgow, 1984).
5. C. Y. Young, R. Pindak, N. A. Clark and R. B. Meyer, Phys. Rev. Lett. 40, 773 (1978).
6. There are even a few reports on films only a single molecular layer in thickness. For example, C. Rosenblatt, R. Pindak, N. A. Clark and R. B. Meyer, Phys. Rev. Lett. 42, 1220 (1979).
7. However, we were not able to observe a single example of such a rare, fragile film in the course of our research.
8. A brief review of recent work investigating surface ordering in liquid-crystal films can be found in R. Geer, T. Stoebe and C. C. Huang, Phys. Rev. E 48, 408 (1993).
9. P. Mach, S. Grantz, D. A. Debe, T. Stoebe and C. C. Huang, J. Phys. (France) II 5, 217 (1995).
10. The thickness of thicker films (N > 20) could be determined to better than 10% by examining the chromaticity of reflected white light and using the coordinates determined by E. B. Sirota, P. S. Pershan, L. B. Sorensen and J. Collet, Phys. Rev. A 36, 2890 (1987).
11. The thickness (in units of molecular layers) of thinner films could be determined exactly using the method developed by C. Rosenblatt and N. Amer, Appl. Phys. Lett. 36, 432 (1980).
12. I. Langmuir, J. Am. Chem. Soc. 38, 2221 (1916).
13. P. Pieranski, L. Beliard, J.-Ph. Tournellec, X. Leoncini, C. Furtlehner, H. Dumoulin, E. Riou, B. Jouvin, J.-P. Fenerol, Ph. Palaric, J. Heuving, B. Cartier and I. Kraus, Physica 194A, 364 (1993).
14. The measured surface tension values were fit to a linear form γ = γ0+ bN where N is the film thickness measured in layers. The fit yielded b = 3.8 times 10-4 dyn/cm layer. The resolution of such a small value was greatly aided by the large range in film thicknesses reported, up to 6000 layers.
15. Our thickness dependence resolution was limited primarily by our inability to create and characterize uniform, thick (> 500 layers) films of sufficient size ( approx 1 cm2). Stresses associated with line energies induced by discontinuous variations in thickness of nonuniform films could alter the effective film tension. Therefore, only results obtained on films of uniform thickness are presented here. However, nonuniform thicker (but still much less than 6000 layers) films exhibited surface tension values consistent with those reported on uniform films to within our estimated error, suggesting a smaller lower limit for the surface tension dependence on film thickness.
16. Because it would involve a tremendous increase in the surface area of the film, noncoplanar terminal groups are unlikely. Furthermore, exposure of the relatively high polarizability CH2 and CF2 groups is also not favorable (see Ref. [14]) and significant conformational alterations near the surface are possible but not expected.
17. W. A. Zisman, Contact Angle, Wettability and Adhesion, edited by F. M. Fowkes, Advances in Chemistry Series No. 43 (American Chemical Society, Washington, D.C., 1964), p. 1. The critical surface energy is closely related to the solid-vapor interfacial tension for systems dominated by dispersive forces.
18. P. G. de Gennes, Rev. Mod. Phys. 57, 827 (1985).
19. J. Israelachvili, Intermolecular and Surface Forces (Academic, London, 1992), p. 94.
20. The fluorocarbon-air and hydrocarbon-air surface free energies have recently been reported as 9.5 and 19.3 dyn/cm, respectively, by Manoj K. Chaudhury and Michael J. Owen, Langmuir 9, 29 (1993).
21. Using these values, the surface free energy of a surface comprised of roughly equal proportions of CF3 and CH3 terminal groups can be estimated as approx (9.5 times 19.3 )1/2= 13.5 dyn/cm, again in good agreement with our experimental results.
22. J. D. Shindler, E. A. L. Mol, A. Shalaginov, and W. H. de Jeu, Phys. Rev. Lett. 74, 722 (1995).
23. An antiparallel structure for these fluorinated compounds has also been proposed by T. P. Rieker and E. P. Janulis, Liq. Cryst. 17, 681 (1994).
24. This figure can be estimated using volumetric data obtained on perfluorinated and nonperfluorinated polymers found in the Polymer Handbook, edited by J. Brandup and E. H. Immergut (J. Wiley and Sons, New York, 1975).