Particle-stabilized defect gel in cholesteric liquid crystals

Science

Volumn 283, Issue 5399, 1999, Pages 209-212

  Zapotocky, Martin ^a   Ramos, Laurence ^a   Poulin, Philippe ^a,b   Lubensky, T C ^a   Weitz, D A ^a  

^a UNIVERSITY OF PENNSYLVANIA   (United States)

^b CENTRE DE RECHERCHE PAUL PASCAL   (France)

Author keywords

[No Author keywords available]

Indexed keywords

COLLOIDS; CRYSTAL DEFECTS; DISPERSIONS; ELASTICITY; RHEOLOGY; TENSILE STRESS;

PARTICLE STABILIZED DEFECT GEL; RHEOLOGICAL MEASUREMENT;

CHOLESTERIC LIQUID CRYSTALS;

CHOLESTEROL;

ARTICLE; CHEMICAL STRUCTURE; COLLOID; FLOW KINETICS; LIQUID CRYSTAL; MOLECULAR DYNAMICS; MOLECULAR INTERACTION; MOLECULAR STABILITY; OPTICAL DENSITY; PRIORITY JOURNAL;

EID: 0033534649     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.283.5399.209     Document Type: Article

References (28)

1. P.-G. de Gennes and J. Prost, The Physics of Liquid Crystals (Clarendon, Oxford, 1993).
2. R. Jowitt et al., Eds., Physical Chemistry of Foods (Applied Science, New York, 1983).
3. L. E. Nielsen and R. F. Landel, Mechanical Properties of Polymers and Composites (Dekker, New York, ed. 2, 1994), chap. 7.
4. film ranging from 20 to 100 μm between microscope slides; thicker films lead to excessive light scattering and preclude the direct observation of their defect structure. The glass slides were coated with rubbed polyimide to enforce a parallel boundary condition at the surface for the nematic director, n, which specifies the average local orientation of the molecules. In this case, the lowest free-energy state of the pure cholesteric is the planar texture (1), characterized by horizontal layers connecting points of equal orientation of n (Fig. 1A). The cholesteric twist axis t in this configuration is oriented perpendicular to the film, and layers of equivalent orientation of n are spaced by the distance h = p/2.
5. P. Poulin and O. Mondain-Monval, unpublished data.
6. M. R. Kamal and A. Mutel, J. Polym. Eng. 5, 293 (1985).
7. R. G. Larson et al., Rheol. Acta 32, 245 (1993); S. Paasch, F. Schambil, M. J. Scwuger, Langmuir 5, 1344 (1989).
8. R. G. Larson et al., Rheol. Acta 32, 245 (1993); S. Paasch, F. Schambil, M. J. Scwuger, Langmuir 5, 1344 (1989).
9. R. H. Colby et al., Rheol. Acta 36, 498 (1997).
10. K. Kawasaki and A. Onuki, Phys. Rev. A 42, 3664 (1990).
11. H. E. Warriner et al., Science 271, 969 (1996); J. Chem. Phys. 107, 3707 (1997); S. L. Keller et al., Phys. Rev. Lett. 70, 4781 (1997).
12. H. E. Warriner et al., Science 271, 969 (1996); J. Chem. Phys. 107, 3707 (1997); S. L. Keller et al., Phys. Rev. Lett. 70, 4781 (1997).
13. H. E. Warriner et al., Science 271, 969 (1996); J. Chem. Phys. 107, 3707 (1997); S. L. Keller et al., Phys. Rev. Lett. 70, 4781 (1997).
14. P. Poulin, H. Stark, T. C. Lubensky, D. A. Weitz, Science 275, 1770 (1997).
15. See, for example, M. Kléman, Points, Lines, and Walls (Wiley, Chichester, UK, 1983).
16. T. C. Lubensky, Phys. Rev. A 6, 452 (1972).
17. _, P. Poulin, L. Ramos, D. A. Weitz, M. Zapotocky, in preparation.
18. P. Boltenhagen, O. Lavrentovich, M. Kléman, J. Phys. II (Paris) 1, 1233 (1991).
19. 2/s. These velocities are reduced (but remain constant in time) if a colloidal inclusion detaches together with the streak, as is sometimes observed, and is dragged through the fluid by the end of the streak. They are also reduced when only a part (of thickness s) of the cross section of a thick oily streak retracts; this process occurred frequently especially in thick films.
20. O. D. Lavrentovich, M. Kléman, V. M. Pergamenshchik, J. Phys. II (Paris) 4, 377 (1994).
21. The oily streak network at zero stress maintains its node and link structure. It is, therefore, more analogous to a fixed-network chemical gel than to a physical gel whose network undergoes dynamical rearrangements.
22. A short video of the moving bubble experiment is available at www.physics.upenn.edu/~weitzlab/research/ cholesteric.html.
23. net.
24. The cholesteric pitch was 15 μm in both cases. The materials were rapidly quenched from the isotropic to the cholesteric phase and maintained at 20°C. We used a double-wall Couette apparatus with gap of 1 mm and measured the frequency dependence of the storage modulus G′(ω) and loss modulus G″(ω) in a small-amplitude oscillatory shear experiment.
25. Without preshearing, G′(ω) and G″(ω) indicated solid-like behavior similar to that reported for pure smectics (7, 8) but with magnitudes of the moduli three to four orders of magnitude smaller. This result is consistent with the presence of a persistent macroscopically disordered texture of cholesteric layers.
26. Recall that a solid is characterized by G′(ω) → const ≠ 0 and G″(ω) → 0 as ω → 0; this implies G′(ω) > G″(ω) at sufficiently low frequencies. In a liquid, in contrast, G′(ω)/G″(ω) → 0 as ω → 0.
27. 2 is the elastic shear modulus in the linear regime, thus providing an internal consistency check of our data.
28. We thank T. Martin for participation in the early stages of this project, V. Trappe for help with rheometry, and O. Lavrentovich for useful discussions. This work was supported primarily through NSF grant DMR95-07366, as well as the bourse Lavoisier du Ministère Français des Affaires Etrangères.