Diffusion in thermotropic liquid crystals

Krüger, G. J.

doi:10.1016/0370-1573(82)90025-4

Cited by 178 publications

(82 citation statements)

References 60 publications

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“…It has been argued that, in NLCs, the self-diffusion of the solvent molecules and the tracer diffusion of a probe molecule of similar size will, in general, show the same basic features, but the anisotropy will be reduced for the latter. 4 Keeping this caveat in mind, it nonetheless remains clear that tracer diffusion is a very helpful tool for improving the understanding of NLC dynamics. FRS tracer diffusion, in particular, provides a very direct experimental probe of the anisotropic micron-scale dynamics of NLC fluids, which remains an area with many open questions.…”

Section: Introductionmentioning

confidence: 99%

Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings

Spiegel

Thompson

Campbell

2001

The Journal of Chemical Physics

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We have employed forced Rayleigh scattering ͑FRS͒ to study the diffusion of an azo tracer molecule ͑methyl red͒ through a nematic liquid crystal ͑5CB͒. This system was first investigated in an important study by Hara et al. ͓Jpn. J. Appl. Phys. 23, 1420 ͑1984͔͒. Since that time, it has become clear that the presence of complementary ground-state and photoproduct FRS gratings can result in nonexponential profiles, and that complementary-grating effects are significant even when ''minor'' deviations from exponential decay are observed. We have investigated the methyl red/5CB system in order to evaluate the possible effects of complementary gratings. In the isotropic phase, we find that the presence of complementary gratings results in a nonmonotonic FRS signal, which significantly changes the values inferred for the isotropic diffusion coefficients. As a result, the previously reported discontinuity at the nematic/isotropic transition temperature (T NI ) is not present in the new data. On the other hand, in the nematic phase, the new experiments largely confirm the previous observations of single-exponential FRS decay and the non-Arrhenius temperature dependence of the nematic diffusion coefficients close to T NI . Finally, we have also observed that the decrease in the diffusion anisotropy with increasing temperature can be correlated with the 5CB nematic order parameter S(T) over the full nematic temperature range.

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Section: Introductionmentioning

confidence: 99%

Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings

Spiegel

Thompson

Campbell

2001

The Journal of Chemical Physics

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“…The ratio is much smaller for these simulated shorter particles (molecules), but still D z /D xy > 1 [18,20] or D z /D xy ≃ 1 [19] in the SmA phase. Experimental studies of thermotropic smectic phases show D xy /D z ≥ 1 [21]. The effect of flexibility of the molecules in hopping-like diffusion in the smectic phase also has been discussed [22].…”

Section: One-dimensional Hopping In Liquid Crystal Phasementioning

confidence: 99%

One-, Two-, and Three-Dimensional Hopping Dynamics

et al. 2013

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Hopping dynamics in glass has been known for quite a long time. In contrast, hopping dynamics in smectic-A (SmA) and hexatic smectic-B (HexB) liquid crystals (LC) has been observed only recently. The hopping in SmA phase occurs among the smectic layers (one-dimensionally), while hopping in HexB phase occurs inside the layers (two-dimensionally). The hopping dynamics in SmA and HexB liquid crystal phases is investigated by parallel soft-core spherocylinders, while three-dimensional hopping dynamics in inherent glassy states is investigated by systems of Weeks-Chandler-Andersen (WCA) spheres. The temperature dependence of diffusion coefficients of hopping in SmA phase can be described by the Arrhenius equation characteristic of activation process. In HexB LC phase, the diffusion coefficients saturate at higher temperatures. In a system of WCA spheres, the values and temperature dependence of diffusion coefficients depend on the observed states.

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“…These observations show an opposite trend as compared to ther-motropic liquid crystals [4,5], where usually D /D ⊥ evolves from being larger than one at temperatures close to the N-Sm transition temperature to being smaller than one at lower temperatures [24]. Therefore the diffusion in the smectic phase can be effectively considered as a onedimensional diffusion of a Brownian particle in a periodic potential in the high friction limit.…”

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confidence: 99%

Self-Diffusion of Rodlike Viruses through Smectic Layers

2007

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We report the direct visualization at the scale of single particles of mass transport between smectic layers, also called permeation, in a suspension of rod-like viruses. Self-diffusion takes place preferentially in the direction normal to the smectic layers, and occurs by quasi-quantized steps of one rod length. The diffusion rate corresponds with the rate calculated from the diffusion in the nematic state with a lamellar periodic ordering potential that is obtained experimentally.Since the pioneering work of Onsager on the entropy driven phase transition to a liquid crystalline state [1], the structure and the phase behavior of complex fluids containing anisotropic particles with hard core interactions has been a subject of considerable interest, both theoretically [2] and experimentally [3]. Understanding of the particle mobility in the different liquid crystalline phases is more recent [4]. In experiments various methods have been applied to obtain the ensemble averaged selfdiffusion coefficients in thermotropic [5] and amphiphilic [6] liquid crystals, block copolymer [7] and colloidal systems [8]. Only a few studies have been done where dynamical phenomena are probed at the scale of a single anisotropic particle: the Brownian motion of an isolated colloidal ellipsoid in confined geometry [9] and the selfdiffusion in a nematic phase formed by rod-like viruses [10] represent two recent examples. In the latter case, the diffusion parallel (D ) and perpendicular (D ⊥ ) to the average rod orientation (the director) has been measured, showing an increase of the ratio D /D ⊥ with particle concentration. Knowledge of the dynamics at the single particle level is fundamental for understanding the physics of mesophases with spatial order like the smectic (lamellar) phase of rod-like particles. In this mesophase the particle density is periodic in one dimension parallel to the long axis of the rods, while the interparticle correlations perpendicular to this axis are short-ranged (fluid-like order). For parallel diffusion to take place, the rods need to jump between adjacent smectic layers, overcoming an energy barrier related to the smectic order parameter [11]. This process of interlayer diffusion, or permeation, was first predicted by Helfrich [12]. In this Letter, we use video fluorescence microscopy to monitor the dynamics of individual labeled colloidal rods in the background of a smectic mesophase formed by identical but unlabeled rods. In this way we directly observe permeation of single rods in adjacent layers. As in the nematic phase, self-diffusion in a smectic phase is anisotropic: the diffusion through the smectic layers is shown here to be much faster than the diffusion within each liquid-like layer, i.e. D /D ⊥ ≫ 1, in contrast to thermotropic systems. Moreover, since the individual rod positions within the layer are monitored, the potential barrier for permeation is straightly determined for the first time. The permeation can then be . The layer spacing is L ≃ 0.9 µm. (b) Displacement of a given particle...

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Diffusion in thermotropic liquid crystals

Cited by 178 publications

References 60 publications

Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings

Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings

One-, Two-, and Three-Dimensional Hopping Dynamics

Self-Diffusion of Rodlike Viruses through Smectic Layers

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