Complex fluid behavior: coupling of the shear stress with order parameter tensors of ranks two and three in nematic liquid crystals and in tetradic fluids

Hess, Siegfried

doi:10.1016/s0378-4371(02)01145-7

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“…13 Threshold diagram for different regimes of roll instability. The ratio of oscillations amplitude to the layer thickness is equal to 0.148; R-Z: the rolls are visualized as the inversion of dark and light stripes every half of the period, which corresponds to the change of a sign for n z projection at a constant sign for n y ; R-Y: there is no inversion of dark and light stripes that correspond to the change of a sign for n y projection at a constant sign for n z ; TR: intermediate regime with an exotic texture.…”

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Flows of Anisotropic Liquids

2009

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j45 crystals, presented in Chapter 2. The connection between translation motions and the orientational ones described accordingly in terms of a velocity field v(r, t) and a director field n(r, t) can be considered as a fundamental physical property of liquid crystals. It is responsible for a number of specific phenomena existing in liquid crystal media. Some of them, such as various instabilities induced by electric fields in thin layers of nematic liquid crystals or backflow effects arising at the turning off electric fields are of great practical importance (Chapter 6).The connection between v(r, t) and n(r, t) also results in a number of linear and nonlinear phenomena in flows of liquid crystals. In general, liquid crystals show non-Newtonian rheological behavior. It means, for example, that apparent shear viscosity depends on the shear rate. At the same time, strong magnetic (electric) fields allow to stabilize orientational structure. In thin layers of LC, the proper surface treatment can also provide the given orientation (e.g., planar or homeotropic, see Chapter 2). Liquid crystal with a stabilized orientational structure can be considered a conventional Newtonian fluid with shear viscosity determined by a field direction [1] or by a surfaceinduced direction. Such rheological behavior has to be taken into account at viscometric studies of liquid crystals. It will be described in detail in Chapter 5.A number of interesting flow-induced phenomena in smectic A and C phases are out of scope of this book. A reader can find description of such effects in Oswald and Pieranski [2] and in original papers [3,4]. We also do not consider rheological behavior of polymeric liquid crystals, lyotropic liquid crystals, and active liquid crystals. The last two classes of LC materials are of interest for understanding very complicated phenomena existing in living nature. In particular, active liquid crystals show unusual rheological properties that demand a special consideration [5][6][7].

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