Abstract
The dynamical properties of a system of linear domains induced by shear oscillations in a planar layer of a nematic liquid crystal under the joint action of shear and piston oscillations at frequencies in the audio range, have been studied experimentally. The phenomenon of spatial modulation of the domains of an initial system along their axis under the piston oscillations producing periodic layer compression is investigated for the case where the directions of the director in a mesophase layer and the shear oscillations coincide. A relationship between the period of secondary two-dimensional structures at the effect threshold and the layer thickness as well as oscillation frequency is established. It is shown that the formation mechanism of these structures can be interpreted on the basis of a known phenomenological model, according to which the instability of the initial system of these linear domains is caused by the “mismatch” between the real period of domains and its optimal value, which varies jointly with the variations in the thickness of a periodically compressed layer.
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Notes
According to the theory [3], under the action of “ideal” shear on an NLC homeotropic layer, its molecules periodically deviate from the normal to layer plane; this deviation manifests itself in polarized light observations as a change in the optical transparency of NLC-containing cell placed between crossed polaroids.
The plates of an NLC-containing cell performs oscillations in the XY plane (Fig. 1a) in orthogonal directions (along the X and Y axes) with a phase shift of π/2. Under these conditions, the NLC director n [5] follows the displacement of plates and moves over an elliptical cone, whose axis coincides with the elliptical shift axis. The system of linear domains arising at the orientational instability threshold is located in the layer plane making an angle with the X axis, whose value is determined by the ratio of the X and Y components of the plate motion ellipse; the width of these domains is proportional to the layer thickness.
According to [12], the domains of existence of different modes of NLC orientational instability in an oscillating Couette flow with a linear velocity profile, initiated by shear oscillations, is separated by a critical angle φcr ~ arctan[1 + (α5/α4 – 2α3/α6)/3]. For MBBA and Leslie viscosity coefficients αi [5] corresponding to 23°С, φcr ≈ 0.77 rad.
It is known that a change in the spatial and temporal order during structural transformations in NLC under an external effect is generally accompanied by generation of defects, which may play an important role, affecting the scenarios of domain formation and turbulization [15].
This selectivity of the action of piston oscillations on the NLC layer is due to the fact that the velocity of spreading nematic liquid in a Poiseuille flow under conditions of periodic layer compression increases while moving from the center of the layer to its edges.
Note that the formation of incommensurate structures in the NLC planar layer under above-threshold conditions was also observed in spatially modulated electric fields; the mechanism of the decrease in the period of flexoelectric domains [15], arising under these conditions, with an increase in the field strength was explained by the presence of dislocations [16].
The wave number qx(α) of distortion takes into account the correction for the geometric factor: decrease in the distance between the domains during their rotation by an angle α relative to the Y axis.
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ACKNOWLEDGMENTS
I am grateful to E.K. Negazin for the help in carrying out experiments and to Ya.V. Kucherenko for the discussion of the results
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Translated by Yu. Sin’kov
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Kapustina, O.A. Particularities of the Dynamics of Linear Domains in Liquid Crystals under a Binary Acoustic Action. Crystallogr. Rep. 64, 443–450 (2019). https://doi.org/10.1134/S1063774519030131
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DOI: https://doi.org/10.1134/S1063774519030131