Direct observation of coupling between orientation and flow fluctuations in a nematic liquid crystal at equilibrium

Orihara, Hiroshi; Sakurai, Nobutaka; Sasaki, Yuji; Nagaya, Takashi

doi:10.1103/physreve.95.042705

Cited by 5 publications

(4 citation statements)

References 18 publications

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“…Typical NLCs consist of rodlike molecules with longrange orientational order, where the average direction of the molecules is designated by a unit vector n that is referred to as the director [17,18]. One of the most remarkable properties of NLCs is the coupling between the director and the flow; a change in the director can induce flow and vice versa [17][18][19]. Furthermore, the director can be controlled by external electric fields owing to the dielectric anisotropy ε = ε −ε ⊥ , where ε and ε ⊥ are the relative dielectric constants parallel and perpendicular to n, respectively.…”

Section: Introductionmentioning

confidence: 99%

Negative viscosity of a liquid crystal in the presence of turbulence

et al. 2019

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We report on the discovery of enormous negative viscosity in a nematic liquid crystal in the presence of turbulence induced by electric fields. As the negative viscosity in this system is so large, we are able to observe several phenomena originating from it. For example, we observe a spontaneous shear flow that rotates the upper disk of a rheometer, as well as the reversal of the rotational direction upon applying an external torque in the opposite direction. Hysteresis loops are also observed in the shear-stress-shear-rate curves, which is reminiscent of those seen for ferromagnetic and ferroelectric materials. The similarities between the phenomena observed for our system and ferroic materials are comprehensively demonstrated, although the two systems are fundamentally different in that the former is out of equilibrium. We elucidate the origin of the negative viscosity and propose a simple model that reproduces the phenomena observed in this active fluid.

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Section: Introductionmentioning

confidence: 99%

Negative viscosity of a liquid crystal in the presence of turbulence

et al. 2019

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“…Liquid crystal (LC) is an intermediate phase between liquid and solid and can be regarded as a complex fluid with the structure characterised by a director distribution. The flow and director fields strongly couple with each other in LC systems under the existence of a flow [28][29][30][31][32][33][34][35]. This indicates the existence of the aforementioned fluid-structure interaction problem.…”

Section: Introductionmentioning

confidence: 99%

Self-excited oscillation of the director field in cholesteric liquid crystalline droplets under a temperature gradient

Yoshioka¹,

Fukao²

2020

J. Phys.: Condens. Matter

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In this study, we demonstrate a self-excited oscillation induced in cholesteric liquid crystalline droplets under a temperature gradient. At equilibrium, a winding Maltese cross pattern with a point defect was observed via polarised microscopy in the droplets dispersed in an isotropic solvent. When the temperature gradient was applied, the pattern was deformed owing to the Marangoni convection induced by the gradient. Here, when both the droplet size and temperature gradient were sufficiently large, the periodic movement of the defect together with the pattern deformation was observed, which demonstrated the self-excited oscillation of the director field. To describe this phenomenon, we theoretically analysed the flow and director fields by using Onsager's variational principle. This principle enabled the simplified description of the phenomenon; consequently, the time evolution of the director field could be expressed by the phenomenological equations for the two parameters characterising the field. These equations represented the van der Pol equation, which well expressed the mechanism of the self-excited oscillation.

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“…We thus expect to reproduce by numerical simulations the observed so-called speckle pattern produced when a laser beam propagates through a liquid-crystal device. In order to compare the results with the literature [15,16], we simulate the propagation of visible light (532 nm) through a planar aligned 50-µm cell filled with E7. The parameters used for E7 are K = 12 pN and ε = 2.9204, ε ⊥ = 2.2681 [28].…”

Section: A Linear Propagationmentioning

confidence: 99%

“…The proper modeling of the director fluctuations is of particular importance for the understanding of phenomena that depend on noise, such as modulation instability [12,13], filamentation [14], or speckle formation [15,16]. Also, the thermally induced refractive index fluctuations are responsible for soliton spatial fluctuations.…”

Section: Introductionmentioning

confidence: 99%

Spatial fluctuations of optical solitons due to long-range correlated dielectric perturbations in liquid crystals

et al. 2017

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Starting from the de Gennes theory of director fluctuations in nematics, we report on a model of the spatial fluctuations in nematicon propagation. We demonstrate that, when the long-range correlation that characterizes nematic liquid crystals is taken into account in the thermal noise, it is possible to account for the spatial oscillations and propagation losses experienced by nematicons. Increasing the power of the nematicon, the oscillation amplitudes increase and the propagation losses decrease. The nematicon is then more strongly confined and deviates more, but is less scattered by the thermally induced perturbations of the refractive index. All the results are in good agreement with the experimental observations.

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Direct observation of coupling between orientation and flow fluctuations in a nematic liquid crystal at equilibrium

Cited by 5 publications

References 18 publications

Negative viscosity of a liquid crystal in the presence of turbulence

Negative viscosity of a liquid crystal in the presence of turbulence

Self-excited oscillation of the director field in cholesteric liquid crystalline droplets under a temperature gradient

Spatial fluctuations of optical solitons due to long-range correlated dielectric perturbations in liquid crystals

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