Enhancement of order fluctuations in a nematic liquid crystal by sonication

Fréedericksz transition in the director-density coupling theory

We show that the director-density coupling theory gives rise to a singular behavior for the mass density. To overcome this drawback, we propose to supplement the theory with a term that can be derived by regarding liquid crystals as anisotropic Korteweg fluids. We thus show that the static bevahior of the resulting theory predicts a Fréedericksz transition accompanied by a modulation in the mass density.

Acousto-optic effect in nematic liquid crystals: experimental evidence of an elastic regime

We show that the experimental data for the action of ultrasonic waves on homeotropically aligned nematic-liquid-crystal cells reported by Kapustina, in Akust. Zh. 54, 900 (2008) [Acoust. Phys. 54, 778 (2008)] can be explained in the framework of the director-density coupling theory in the regime of low acoustic intensity. This result therefore provides support for the hypothesis that the interaction between sound and nematic liquid crystals is dominated by an elastic energy.

Low-frequency oscillations of the director in nematic liquid crystals induced by ultrasonic waves

The director-density coupling theory has been proposed to describe the acousto-optic effect in nematic liquid crystals. On the basis of this theory, we make predictions for an experimental test using two superimposed ultrasonic waves. As a result of the analysis, low-frequency oscillations of the director and optical transparency are predicted to occur around their mean values. The possibility of either verifying or invalidating this prediction is important in order to distinguish the theory from other competitive ones.

Director fluctuations in nematic liquid crystals induced by an ultrasonic wave

The director-density coupling theory was formulated with two parameters (u(1) and u(2)) to explain the acousto-optic effect in nematic liquid crystals. The assumption that the director is not able to accompany rapid oscillations of the sound wave, so that it actually couples to the time-averaged interaction, renders it effectively a u(1)-independent theory. In this paper, we investigate a route in which the time average is postponed to the end of the calculation. This approach allows us to derive measurable quantities that depend on both u(1) and u(2).

Director-density coupling theory of the acousto-optic effect in nematic liquid crystals

Experiments with nematic liquid crystals have proved that an ultrasonic wave exerts a torque on the liquid-crystal molecules, causing a change in its optical properties (acousto-optic effect). In this work we report a theoretical study on the theory proposed by Selinger et al. [Phys. Rev. E 66, 051708 (2002).] and, independently, by Boneto et al. [Chem. Phys. Lett. 361, 237 (2002).] for this effect. We solved exactly the Euler-Lagrange equation, which determines the equilibrium configuration of the director profile. The liquid-crystal director is also calculated in powers of the acoustic intensity and a comparison of this expansion with the solution in a closed form is given. We show the existence of minimizers that does not satisfy the Euler-Lagrange equation and report the possibility of observing a Fréedericksz-type transition. Finally, a possibility of controlling light by ultrasonic wave is also discussed in the limit of low acoustic intensity.

Director libration in nematoacoustics

We extended the analysis of a variational theory for nematoacoustics recently proposed by Virga [Phys. Rev. E 80, 031705 (2009)] by allowing the nematic director to vibrate about an average orientation at the frequency of a propagating wave, a periodic motion that we call the director libration. The acoustic susceptibilities, two phenomenological parameters that, in this theory, express the coupling between director and acoustic fields, are estimated along with an extra viscosity coefficient by using available experimental data.

Variational theory for nematoacoustics

The effect of an ultrasonic wave on the nematic texture has long been known, but its interpretation in terms of a coherent dynamical theory has not yet been achieved. A proposal for such a theory is made in this paper. The diverse theoretical approaches attempted in the past to describe the interaction between sound and nematic molecular orientation are briefly summarized. A theory for second-grade fluids, which provides the appropriate theoretical background for nematoacoustics, is also revived. An explicit application of the proposed theory to a simple computable case is given, which yields predictions that are qualitatively confirmed by a number of experimental results.

Vibrational Fréedericksz transition in liquid crystals

Our aim is to show the possibility of a "vibrational Fréedericksz's transition" in nematic liquid crystals (LCs) consisting of asymmetric molecules (in the absence of the reflectional symmetry for the director field). For this purpose we study the effects of the external high-frequency vibrations imposed on LCs by employing the rigorous two-timing asymptotic averaging method to the governing LC equations in their most general form. We restrict our attention only to translational mechanical vibrations of an incompressible LC medium as a whole, so our vibrations are not related to acoustical waves. We show that the averaged "vibrogenic" torque acted on the director field is mathematically equivalent to the torque caused by an external magnetic field. This equivalence leads us to the key conclusion that the high-frequency mechanical vibrations can cause the vibrational Fréedericksz's transition. Our evaluation of the relevant physical parameters shows that this phenomenon can be observed in laboratory experiments.

Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication

Proton field-cycling nuclear magnetic resonance relaxometry is used to study the spin-lattice relaxation dispersion of selected standard smectic A liquid crystals at different temperatures. Relaxation features at both, in the presence and absence of a monochromatic ultrasonic field are considered. We show that the laboratory-frame spin-lattice relaxation time is mainly governed by translational diffusion. Order director fluctuations (ODF) are less important while rotational diffusion seems to be only relevant near the clearing point. Our study suggests that sonication enhances the ODF contribution in the SmA mesophase. Within the framework of the approach we have outlined, different features associated with the ODF mechanism can be investigated.

Apparent low-field spin-lattice dispersion in the smectic-A mesophase of thermotropic cyanobiphenyls

Proton field-cycling spin-lattice relaxometry T1 of the smectic-A mesophase in cyanobiphenyls revealed the presence of steep dispersions in the low-frequency regime. We clearly show that the strong dispersion characteristic of smectic organizations cannot be attributed to the collective molecular dynamics (order director fluctuations), as it is usually interpreted. We present two independent experimental evidences: the dependence of the dispersion with the slew rate of the magnetic field cycle and the dependence of the dispersion with the presence and power of an ultrasonic field.

Spin-lattice dispersion in nematic and smectic-A mesophases in the presence of ultrasonic waves: a theoretical approach

We present a theoretical study of the Larmor frequency dependence of the nuclear spin-lattice relaxation caused by order director fluctuations for both nematic and smectic-A mesophases. The analysis is focused on the case where the molecular system is subjected to sonication during the relaxation process. The departure from the nonsonicated case is discussed for various values of the involved parameters. Two different approaches are discussed for the smectic case.