Director dynamics in liquid-crystal physical gels

Nematic liquid-crystal (LC) elastomers and gels have a rubbery polymer network coupled to the nematic director. While LC elastomers show a single, non-hydrodynamic relaxation mode, dynamic light-scattering studies of self-assembled liquid-crystal gels reveal orientational fluctuations that relax over a broad time scale. At short times, the relaxation dynamics exhibit hydrodynamic behavior. In contrast, the relaxation dynamics at long times are non-hydrodynamic, highly anisotropic, and increase in amplitude at small scattering angles. We argue that the slower dynamics arise from coupling between the director and the physically associated network, which prevents director orientational fluctuations from decaying completely at short times. At long enough times the network restructures, allowing the orientational fluctuations to fully decay. Director dynamics in the self-assembled gels are thus quite distinct from those observed in LC elastomers in two respects: they display soft orientational fluctuations at short times, and they exhibit at least two qualitatively distinct relaxation processes

The flashing ratchet and unidirectional transport of matter

We study the flashing ratchet model of a Brownian motor, which consists in cyclical switching between the Fokker-Planck equation with an asymmetric ratchet-like potential and the pure diffusion equation. We show that the motor really performs unidirectional transport of mass, for proper parameters of the model, by analyzing the attractor of the problem and the stationary vector of a related Markov chain.Comment: 11 page

Lorentz-Lorenz Coefficient, Critical Point Constants, and Coexistence Curve of 1,1-Difluoroethylene

We report measurements of the Lorentz-Lorenz coefficient density dependence, the critical temperature, and the critical density, of the fluid 1,1-difluoroethylene. Lorentz-Lorenz coefficient data were obtained by measuring refractive index and density of the same fluid sample independently of one another. Accurate determination of the Lorentz-Lorenz coefficient is necessary for transformation of refractive index data into density data from optics-based experiments on critical phenomena of fluid systems done with different apparatus, with which independent measurement of the refractive indes and density is not possible. Measurements were made along the coexistence curve of the fluid and span the density range 0.01 to 0.80 g/cc. The Lorentz-Lorenz coefficient results show a stronger density dependence along the coexistence curve than previously observed in other fluids, with a monotonic decrease from a density of about 0.2 g/cc onwards, and an overall variation of about 2.5% in the density range studied. No anomaly in the Lorentz-Lorenz coefficient was observed near the critical density. The critical temperature is measured at Tc=(302.964+-0.002) K (29.814 C) and the measured critical density is (0.4195+-0.0018)g/cc.Comment: 14 pages, 6 figures, MikTeX 2.4, submitted to Physical Review

Optical, magnetic and dielectric properties of non-liquid crystalline elastomers doped with magnetic colloids

Magnetic nanoparticles from magnetic colloidal suspensions were incorporated in the urethane/urea elastomer (PU/PBDO) by swelling fully crosslinked elastomer samples with a toluene and ferrofluid mixture. It is shown that ferrofluid grains can be efficiently incorporated into the matrix of elastomers. The dependence of the birefringence of both the pure and ferrofluid-doped elastomer samples on strain is linear. The ratio of birefringence to strain of the ferrofluid-doped samples is greater than that of the pure elastomer samples, indicating that ferrofluid grains are oriented by the strained polymer network. We propose that this strain-induced orientation is due to the shape anisotropy of the nanoparticles.publishersversionpublishe

n-atic Order and Continuous Shape Changes of Deformable Surfaces of Genus Zero

We consider in mean-field theory the continuous development below a second-order phase transition of $n$-atic tangent plane order on a deformable surface of genus zero with order parameter $\psi = \langle e^{i n \theta} \rangle$. Tangent plane order expels Gaussian curvature. In addition, the total vorticity of orientational order on a surface of genus zero is two. Thus, the ordered phase of an $n$-atic on such a surface will have $2n$ vortices of strength $1/n$, $2n$ zeros in its order parameter, and a nonspherical equilibrium shape. Our calculations are based on a phenomenological model with a gauge-like coupling between $\psi$ and curvature, and our analysis follows closely the Abrikosov treatment of a type II superconductor just below $H_{c2}$.Comment: REVTEX, 12 page

Fluctuating-friction molecular motors

We show that the correlated stochastic fluctuation of the friction coefficient can give rise to long-range directional motion of a particle undergoing Brownian random walk in a constant periodic energy potential landscape. The occurrence of this motion requires the presence of two additional independent bodies interacting with the particle via friction and via the energy potential, respectively, which can move relative to each other. Such three-body system generalizes the classical Brownian ratchet mechanism, which requires only two interacting bodies. In particular, we describe a simple two-level model of fluctuating-friction molecular motor that can be solved analytically. In our previous work [M.K., L.M and D.P. 2000 J. Nonlinear Opt. Phys. Mater. vol. 9, 157] this model has been first applied to understanding the fundamental mechanism of the photoinduced reorientation of dye-doped liquid crystals. Applications of the same idea to other fields such as molecular biology and nanotechnology can however be envisioned. As an example, in this paper we work out a model of the actomyosin system based on the fluctuating-friction mechanism.Comment: to be published in J. Physics Condensed Matter (http://www.iop.org/Journals/JPhysCM

Ferrofluids as thermal ratchets

Colloidal suspensions of ferromagnetic nano-particles, so-called ferrofluids, are shown to be suitable systems to demonstrate and investigate thermal ratchet behavior: By rectifying thermal fluctuations, angular momentum is transferred to a resting ferrofluid from an oscillating magnetic field without net rotating component. Via viscous coupling the noise driven rotation of the microscopic ferromagnetic grains is transmitted to the carrier liquid to yield a macroscopic torque. For a simple setup we analyze the rotation of the ferrofluid theoretically and show that the results are compatible with the outcome of a simple demonstration experiment.Comment: 4 pages, 3 figures, corrected version, improved figures, to be published in Phys. Rev. Let

Driven electronic bridge processes via defect states in 229^{229}Th-doped crystals

The electronic defect states resulting from doping $^{229}$Th in CaF$_2$ offer a unique opportunity to excite the nuclear isomeric state $^{229m}$Th at approximately 8 eV via electronic bridge mechanisms. We consider bridge schemes involving stimulated emission and absorption using an optical laser. The role of different multipole contributions, both for the emitted or absorbed photon and nuclear transition, to the total bridge rates are investigated theoretically. We show that the electric dipole component is dominant for the electronic bridge photon. In contradistinction, the electric quadrupole channel of the $^{229}$Th isomeric transition plays the dominant role for the bridge processes presented. The driven bridge rates are discussed in the context of background signals in the crystal environment and of implementation methods. We show that inverse electronic bridge processes quenching the isomeric state population can improve the performance of a solid-state nuclear clock based on $^{229m}$Th

Electrically tunable laser based on oblique heliconical cholesteric liquid crystal

Acknowledgments We are grateful to V. A. Belyakov and S. V. Shiyanovskii for useful discussions and to G. Cukrov for the measurements of refractive indices. CB9CB was synthesized by the Organic Synthesis Facility at the Liquid Crystal Institute, Kent State University. This work was supported by National Science Foundation DMR 1410378.Peer reviewedPublisher PD