Analysis of dielectric relaxation phenomena with molecular orientational ordering in monolayers at the liquid-air interface

Starting from the Debye theory of rotational Brownian motion, we analyzed the dielectric relaxation phenomena with molecular orientational ordering in Langmuir film on a water surface, assuming that the orientational distribution of the constituent rodlike polar molecules is ruled by Boltzmann statistics. The transient behavior of the polar molecules induced by the application of step compression at the molecular area A=A(i)(<A(c)) is discussed with taking account of the friction constant of monolayer xi, and the dielectric relaxation time tau. Here A(c) is the critical molecular area defined as pi l(2) (l: the length of rodlike polar molecule). The orientational order parameter S of monolayers and Maxwell displacement current (MDC) at the dielectric relaxation process were obtained, for the first time, with the consideration of xi and tau. On the basis of the analysis developed here, we examined the generation of MDC across 4-cyano-4'-5-alkyl-biphenyl (5CB) Langmuir film by monolayer compression

Polar orientational phase transition in smectic monolayers induced by monolayer

Taking into consideration the mean electric field produced by a molecular configuration, a self-consistent-field state equation is derived to describe the behavior of polar smectic phase for monolayers. A general equation of orientational phase transition between a normal-director phase and a tilted-director phase induced by monolayer compression for monolayers is obtained. It was revealed that smectic monolayers show a possible second-order orientational phase transition under a necessary condition 4-16/g(y)<g(z)<g(y)/4, Here g(y) and g(z) are two parameters proportional to the internal electric fields in the directions parallel and perpendicular to the monolayer surface, respectively. The two-dimensional orientational phase transition theory developed here may reveal such a transition in smectic monolayers by some techniques, for example, the Maxwell-displacement-current measuring technique

Analysis of dielectric relaxation time of organic monolayer films on a material surface

Based on the Debye theory of rotational Brownian motion, we derived the dielectric relaxation time tau of monolayers on a material surface in a generalized form. tau was found to be proportional to the orientational fluctuation of monolayers [=[(cos theta-[cos theta])(2)]] (theta is the tilt angle of the constituent polar molecules). Here [cos theta] represents the molecular orientational order parameter of monolayers, and [] represents the thermal average. In this analysis, the Coulomb attractive force working between polar molecules and the material surface as well as the interaction working among molecules were taken into consideration. It was revealed that the interaction working on molecules makes a significant contribution to reduce the relaxation time whether the interaction is attractive or repulsive

Analysis of compression-induced chiral phase separation in Langmuir monolayers

We analyze the compression-induced chiral phase separation (CPS) in Langmuir films, taking into account the elastic theory of liquid crystals and the mixing energy of the two constituent enantiomers. The difference between the Selinger-Wang-Bruinsma-Knobler theory [J. V. Selinger et al., Phys. Rev. Lett. 70, 1139 (1993)] and our treatment is that we do not introduce the concentration-square-gradient term in the free energy, but alternatively take into account a line tension at CPS boundaries. Our model predicts that a two-domain pattern with opposite chiralities is energy minimized, but a multistripe pattern with two alternate constant chiralities is also possible, though metastable. This offers a tentative explanation for the CPS pattern consisting of homogeneously oriented stripes with diverse widths observed by Eckhardt et al

Analysis of weak-anchoring effect in nematic liquid crystals

A generalized Rapini-Papoular-type anchoring energy formula [J. Phys. (Paris) Colloq. 30, C4-54 (1969)] with two coupling constants is established through a second-order spherical-harmonic expansion. Using this formula, we analyze the threshold and saturation properties of twisted nematic devices with unidirectional planar anchorage, assuming that the azimuthal and polar anchoring strengths are both finite and distinct from each other. We also discuss the voltage-controlled-twist effect [G. P. Bryan-Brown et al, Nature (London) 392, 365 (1998)]. It is shown that the predicted behavior is consistent with the experimental observations

Calculation of molecular orientational order and Maxwell displacement current of biaxial molecules at the air-liquid interface

On the basis of the electrodynamic interaction of biaxial molecules (with their larger permanent dipoles perpendicular to the molecular axes) with a liquid surface, we calculate the molecular orientational order and the Maxwell displacement current (MDC) of biaxial molecules with a thermostatistical approach in the range of molecular area of immeasurably low surface pressure. It is shown that MDC generated by orientational change of the permanent dipoles along long molecular axes dominates the compression process shortly after the onset of the transition at A(0) from the planar surface alignment phase to the polar one, despite their smaller dipole moments than those perpendicular to the long molecular axes. This agrees with the fact that the MDC should change with the dipole moment along the long molecular axis. The results also reveal the thermodynamic motions of the dipoles parallel and perpendicular to the long molecular axis during the compression process

Calculation of the dielectric constant of monolayers with dielectric anisotropy with a consideration of molecular configuration

The dielectric constant of a monolayer with dielectric anisotropy on a material surface is calculated, assuming that a two-dimensional array is formed and the orientational distribution of rod-like molecules is ruled by Boltzmann statistics. The theoretical local electric field acting on dipoles is taken into consideration in this calculation. It is found that at the molecule density where the maximum tilt angle is theta(A) = theta(k) = root 3-1)/2, i.e. A/A(0) = root 3/2 (A(0) = pi l(2), l is the length of the long axis of rod-like molecule), the dielectric anisotropic monolayer films degenerate to dielectric-isotropic films with an apparent electronic polarizability = (alpha(parallel to) + (alpha(perpendicular to))/2, regardless of molecular configuration. Under the mean-field approximation the biaxial molecules degenerate to uniaxial ones with an apparent dipole moment mu cos theta(D) and the dependence of dielectric constant on g (see Eq. (9)) is found to be linear. (C) 1998 Elsevier Science S.A. All rights reserved

软物质体系动力学特性及生物体系物质与信号的传输

简要介绍理论物理研究所在软物质领域包括在聚合物刷、蛋白质扩散动力学、复杂流体的相分离方面的研究进展,以及在生物物理方面包括细胞内钙信号和艾滋病免疫系统动力学的研究进展,显示了在软物质及生物物理领域中非线性、多尺度、涨落、熵驱动的重要特征

How proteins squeeze through polymer networks: a Cartesian lattice study

In this paper a lattice model for the diffusional transport of particles in the interphase cell nucleus is proposed. The dynamic behaviour of single chains on the lattice is investigated and Rouse scaling is verified. Dynamical dense networks are created by a combined version of the bond fluctuation method and a Metropolis Monte Carlo algorithm. Semidilute behaviour of the dense chain networks is shown. By comparing diffusion of particles in a static and a dynamical chain network, we demonstrate that chain diffusion does not alter the diffusion process of small particles. However, we prove that a dynamical network facilitates the transport of large particles. By weighting the mean square displacement trajectories of particles in the static chain network data from the dynamical network can be reconstructed. Additionally, it is shown that subdiffusive behaviour of particles on short time scales results from trapping processes in the crowded environment of the chain network. In the presented model a protein with 30 nm diameter has an effective diffusion coefficient of 1.24E-11 m^2/s in a chromatin fiber network.Comment: submitted to J. Chem. Phy

Weak boundary anchoring, twisted nematic effect, and homeotropic to twisted-planar transition

Expansion analysis shows that in second order, the weak boundary coupling of nematic liquid crystals should be depicted by two anchoring coefficients and an orthonormal vector triplet. Using this binomial anchoring energy, we have derived the analytical expression of the threshold and saturation properties of the twisted nematic effect and the homeotropic to twisted-planar transition. Our results prove clearly that these two quite different transitions are reverse effects of each other