Dynamics of Nematic Liquid Crystal under Oscillatory Flow: Influence of Surface Viscosity

Nasibullayev, I.Sh.; Krekhov, A. P.; Khazimullin, M. V.

doi:10.1080/10587250008023290

Cited by 4 publications

(4 citation statements)

References 10 publications

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“…The peak-to-peak displacement of the shearing probe tip was 100 nm and the oscillation frequency was 1 kHz in this study. If we compare these shearing conditions with analyses conducted in previous studies, − it can be seen that they correspond to relatively large disturbance of the weakly aligned LC. Thus, we assume that the molecular orientations were primarily dominated by the flow alignment.…”

Section: Discussionmentioning

confidence: 79%

“…This speculation does not contradict our results, in which the S values were almost identical for shearing without photoalignment, parallel shearing, and perpendicular shearing at h values of 100–500 nm. However, observation of shear thinning in the case of parallel shearing has not been reported in these previous studies examining oscillatory shear flow. − We believe this is because of the high shear rates of 10 3 to 10 4 s –1 expected at the surface layer, which were not achieved in other measurements.…”

Section: Discussionmentioning

confidence: 80%

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Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation

et al. 2015

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In the context of the use of liquid crystals (LCs) as lubricants and lubricant additives, this study investigates the anisotropic shear viscosity of LCs confined in nanometer-sized gap widths subject to both shearing and photoalignment. The photoalignment is achieved using anisotropically dimerized polyvinyl cinnamate (PVCi) films coated on substrates. We simultaneously measure the viscosity and order parameter of a liquid crystal (4-cyano-4'-pentylbiphenyl) confined and sheared in the gap range of 500 nm down to a few nm. We achieve this simultaneous measurement using an original method that combines a highly sensitive viscosity measurement and a sensitive birefringence measurement. When the LC is sheared in the same direction as the photoalignment (parallel shearing), the order parameter, which is around 0.3 in the bulk state, increases up to around 0.4 at a gap width of less than 50 nm and the viscosity is smaller than half the bulk viscosity. We consider that this increase in the order parameter is due to the highly ordered photoaligned LC layer near the PVCi film, and the viscosity decrease is due to shear thinning of this layer enhanced by both confinement and molecular ordering. In addition, we observe a gradual decrease in viscosity starting at a gap of less than around 300 nm in the parallel shearing. Based on the apparent slip model, we show that the LC layer near the PVCi film can also cause this gradual viscosity decrease. In contrast, when the LC is sheared in the direction perpendicular to the photoalignment direction (perpendicular shearing), the viscosity increases as the gap decreases. We speculate that this is due to the rotational motion of the LC molecules caused by the competing effect of shear alignment and photoalignment. We believe our findings can significantly contribute to a better understanding of the confined LCs utilized for lubrication.

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Section: Discussionmentioning

confidence: 79%

Section: Discussionmentioning

confidence: 80%

Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation

et al. 2015

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“…The theoretical description taking into account weak anchoring was developed in the past decade [69,[77][78][79][80][81][82]. It is obvious that the effects of weak anchoring (see Chapter 5) have to modify both static and dynamic behavior of liquid crystals as in the case of electric fields.…”

Section: Shear Flows At Weak Anchoringmentioning

confidence: 99%

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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“…The anchoring characteristics can also be studied in orientational phenomena induced by hydrodynamic flow. In fact, recently the surface orientational transition caused by oscillatory shear flow was found [2] and the influence of weak anchoring on the linear response of the NLC to oscillatory flows was studied [3]. Up to now the study of orientational bulk instabilities in NLCs under hydrodynamic flow have been restricted to the case of strong surface anchoring (fixed director orientation at the confining plates) [4][5][6].…”

Section: Introductionmentioning

confidence: 99%

Influence of weak anchoring on flow instabilities in nematic liquid crystals

Tarasov¹,

Krekhov²,

Kramer³

2001

Liquid Crystals

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We analyse the homogeneous instabilities in a nematic liquid crystal subjected to plane steady Couette or Poiseuille flow in the case when the director is prealigned perpendicular to the flow plane taking into account weak anchoring at the confining surfaces. The critical shear rate decreases for decreasing anchoring strength and goes to zero in the limit of torque-free boundary conditions. For Poiseuille flow one has two types of instability depending on the values of the azimuthal (W a ) and polar (W p ) surface anchoring strengths. The critical line in (W a , W p ) space which separates the two instabilities regimes is obtained.

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Dynamics of Nematic Liquid Crystal under Oscillatory Flow: Influence of Surface Viscosity

Cited by 4 publications

References 10 publications

Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation

Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation

Flows of Anisotropic Liquids

Influence of weak anchoring on flow instabilities in nematic liquid crystals

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