Conductive and dielectric defects, and anisotropic and isotropic turbulence in liquid crystals: Electric power fluctuation measurements

Characterization of surface anchoring energy of nematic liquid crystals via electrohydrodynamic instability

Herein, a method is proposed to determine the azimuthal anchoring energies of surface liquid crystals (LCs), as they gradually change orientation from a vertical to a horizontal state owing to an increase in the voltage applied to each LC cell. The LC cells are characterized using the direction of the Williams roll pattern related to the midplane LC director of the conduction regime of the electrohydrodynamic convection patterns of LCs. The application of the midplane LC directions, obtained from the direction of the roll patterns, to the Ericksen-Leslie equation produces the precise values of the surface anchoring strength. The hybrid type 90°-twisted nematic LC cell, composed of homeotropic and homogeneous LC alignment layers on the top and bottom substrates, respectively, was used to find the azimuthal anchoring energy of the surface LCs, indicated by voltages at the initially vertically aligned LC state. It was observed that the surface azimuthal anchoring energy on the homeotropic layer increased with an increase in voltage. We expect that the proposed technique may be excellent in terms of ease of use, simplicity, and accuracy because the azimuthal anchoring energy can be visually evaluated through the roll pattern.

Negative viscosity of a liquid crystal in the presence of turbulence

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.

Visual evaluation of surface anchoring strength by electrohydrodynamic convection of a nematic liquid crystal

Visual evaluation of the surface anchoring energies in a nematic liquid crystal (LC) cell is characterized by the direction of the convection roll pattern that appears in the low-frequency conduction regime. The convection roll pattern in a twisted nematic LC (TNLC) cell is oriented perpendicular to the midplane LC director dominating the direction of convection flow, and its direction is determined by the relative surface anchoring energy between two surface boundaries. Thus the direction of the roll pattern generated at the TNLC cell with asymmetric LC alignment layers can provide information on the surface anchoring energies at the two boundaries. We demonstrate a method for determining the two anchoring energies through a measured midplane LC director applied to the Ericksen-Leslie equation.

Experimental quenching of harmonic stimuli: universality of linear response theory

We show that liquid crystals in the weak turbulence electroconvective regime respond to harmonic perturbations with oscillations whose intensity decay with an inverse power law of time. We use the results of this experiment to prove that this effect is the manifestation of a form of linear response theory (LRT) valid in the out-of-equilibrium case, as well as at thermodynamic equilibrium where it reduces to the ordinary LRT. We argue that this theory is a universal property, which is not confined to physical processes such as turbulent or excitable media, and that it holds true in all possible conditions, and for all possible systems, including complex networks, thereby establishing a bridge between statistical physics and all the fields of research in complexity.

Event-driven power-law relaxation in weak turbulence

We characterize the spectral properties of weak turbulence in a liquid crystal sample driven by an external electric field, as a function of the applied voltage, and we find a 1/f noise spectrum S(f) proportional, variant1/f;{eta} within the whole range 0<eta<2. We theoretically explore the hypothesis that the system complexity is driven by non-Poisson events resetting the system through creation and annihilation of coherent structures, retaining no memory of previous history (crucial events). We study the time asymptotic regime by means of the density psi(tau) of the time distances between two crucial events, yielding eta=3-micro, where micro is defined through the long-time form psi(tau) proportional, variant1/tau;{micro}, with 1<micro<3. The system regression to equilibrium after an abrupt voltage change experimentally confirms the theory, proving violations of the ordinary linear response theory for both eta>1 and eta<1.

Amplitude modulation and relaxation-oscillation of counterpropagating rolls within a broken-symmetry laser-induced electroconvection strip

We report a liquid-crystal pattern-formation experiment in which we break the lateral (translational) symmetry of a nematic medium with a laser-induced thermal gradient. The work is motivated by an improved measurement (reported here) of the temperature dependence of the electroconvection threshold voltage in planar-nematic 4-methoxybenzylidene-4-butylaniline. In contrast with other broken-symmetry-pattern studies that report a uniform drift, we observe a strip of counterpropagating rolls that collide at a sink point, and a strong temporally periodic amplitude modulation within a width of 3-4 rolls about the sink point. The time dependence of the amplitude at a fixed position is periodic but displays a nonsinusoidal relaxation-oscillation profile. After reporting experimental results based on spacetime contours and wave number profiles, along with a measurement of the change in the drift frequency with applied voltage at a fixed control parameter, we propose some potential guidelines for a theoretical model based on saddle-point solutions for Eckhaus-unstable states and coupled complex Ginzburg-Landau equations.