arXiv subjects

A. D. Kiselev

Publications and source records attributed to A. D. Kiselev.

9 recordsLinked to original sources

arXiv2004

Twist of cholesteric liquid crystal cells: stability of helical structures and anchoring energy effects

We consider helical configurations of a cholesteric liquid crystal (CLC) sandwiched between two substrates with homogeneous director orientation favored at both confining plates. We study the CLC twist wavenumber $q$ characterizing the helical structures in relation to the free twisting number $q_0$ which determines the equilibrium value of CLC pitch, $P_0=2π/q_0$. We investigate the instability mechanism underlying transitions between helical structures with different spiral half-turn numbers. Stability analysis shows that for equal finite anchoring strengths this mechanism can be dominated by in-plane director fluctuations. In this case the metastable helical configurations are separated by the energy barriers and the transitions can be described as the director slippage through these barriers. We extend our analysis to the case of an asymmetric CLC cell in which the anchoring strengths at the two substrates are different. The asymmetry introduces two qualitatively novel effects: (a) the intervals of twist wavenumbers representing locally stable configurations with adjacent helix half-turn numbers are now separated by the instability gaps; and (b) sufficiently large asymmetry, when the difference between azimuthal anchoring extrapolation lengths exceeds the thickness of the cell, will suppress the jump-like behaviour of the twist wavenumber.

cond-mat.soft↗

arXiv2004

Ordering of droplets and light scattering in polymer dispersed liquid crystal films

We study the effects of droplet ordering in initial optical transmittance through polymer dispersed liquid crystal (PDLC) films prepared in the presence of an electrical field. The experimental data are interpreted by using a theoretical approach to light scattering in PDLC films that explicitly relates optical transmittance and the order parameters characterizing both the orientational structures inside bipolar droplets and orientational distribution of the droplets. The theory relies on the Rayleigh-Gans approximation and uses the Percus-Yevick approximation to take into account the effects due to droplet positional correlations.

cond-mat.mtrl-sci↗

arXiv2003

Saddle-splay term induced orientational instability in nematic liquid crystal cells and director fluctuations at substrates

We analyze stability of the planar orientational structure in a nematic liquid crystal (NLC) cell with planar anchoring conditions at both substrates. Specifically, we study the instabilities of the ground state caused by surface elasticity at large saddle-splay elastic constant, $K_{24}$. For relatively small $K_{24}$ violating the Ericksen inequalities the theory predicts that the critical fluctuation mode of the wavelength, $λ_c$, will render the structure unstable when the thickness of the cell is below its critical value, $d_c$. The parity of the critical mode changes as the twist-splay ratio $K_2/K_1$ is passing through the unity. Further increase of $K_{24}$ beyond the second threshold value, $4K_1K_2/(K_1+K_2)$, leads to the instability with respect to the short wavelength fluctuations regardless of the cell thickness. We compute the critical thickness and the critical wavelength as functions of $K_{24}$, the twist-splay ratio and the azimuthal anchoring strength.

cond-mat.soft↗

arXiv2003

Photoinduced 3D orientational order in side chain liquid crystalline azopolymers

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced 3D orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, correspondingly. Using irradiation with the polarized light of two different wavelengths we find that the kinetics of photoinduced anisotropy can be dominated by either photo-reorientation or photoselection mechanisms depending on the wavelength. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.

cond-mat.mtrl-sci↗

arXiv2002

Twist of cholesteric liquid crystal cell with substrates of different anchoring strengths

We consider director configurations of cholesteric liquid crystal (CLC) cells with two plane confining substrates. Exact solutions of the Euler-Lagrange equations for out-of-plane orientations of the easy axes that correspond to inhomogeneous conical structures of CLC director are derived. We study dependence of the CLC twist wavenumber on the free twisting number assuming that anchoring energies at the substrates are either equal or different. In both cases this dependence is found to be generally discontinuous with hysteresis loops and bistability effects involved. For CLC cells with identical substrates and planar anchoring conditions the jump-like behaviour only disappears in the weak anchoring limit. Contrastingly, when the anchoring strengths are different, there is the finite value of anchoring below which the dependence becomes continuous. Another effect is the appearance of the gap between the adjacent twist wavenumber intervals representing locally stable director configurations. We calculate the critical value of anchoring asymmetry and present the results of numerical calculations.

cond-mat.mtrl-sci↗

arXiv2002

Kinetics of photoinduced anisotropy in azopolymers: models and mechanisms

We consider the effect of photoinduced optical anisotropy (POA) in azopolymers. Using a unified approach to the kinetics of photo-reorientation we discuss the assumptions underlying the known theoretical models of POA and formulate a tractable phenomenological model in terms of angular redistribution probabilities and order parameter correlation functions. The model takes into account biaxiality effects and long term stability of POA in azopolymers. It predicts that under certain conditions two different mechanisms, photoorientation and photoselection, will dominate POA depending on the wavelength of pumping light. By using available experimental data, we employ the model to compute dependencies of principal absorption coefficients on the illumination time. Our calculations clearly indicate the different regimes of POA and the numerical results are found to be in good agreement with the experimental data.

cond-mat.mtrl-sci↗

arXiv2001

Light scattering by optically anisotropic scatterers II: T--matrix computations for radially and uniformly anisotropic droplets

This is the second paper in a series on light scattering from optically anisotropic scatterers embedded in an isotropic medium. The apparently complex T-matrix theory involving mixing of angular momentum components turns out to be an efficient approach to calculating scattering in these systems. We present preliminary results of numerical calculations of the scattering by spherical droplets in some simple cases. The droplets contain optically anisotropic material with local radial or uniform anisotropy. We concentrate on cases in which the scattering is due only to the local optical anisotropy within the scatterer. For radial anisotropy we find non-monotonic dependence of the scattering cross-section on the degree of anisotropy can occur in a regime for which both the Rayleigh and semi-classical theories are inapplicable. For uniform anisotropy the cross-section is strongly dependent on the angle between the incident light and the optical axis, and for larger droplets this dependence is non-monotonic.

physics.optics↗

arXiv2001

Light scattering by optically anisotropic scatterers I: T--matrix theory for radial and uniform anisotropies

We extend the T-matrix approach to light scattering by spherical particles to some simple cases in which the scatterers are optically anisotropic. Specifically we consider cases in which the spherical particles include radially and uniformly anisotropic layers. We find that in both cases the T-matrix theory can be formulated using a modified T-matrix ansatz with suitably defined modes. In a uniformly anisotropic medium we derive these modes by relating the wave packet representation and expansions of electromagnetic field over spherical harmonics. The resulting wave functions are deformed spherical harmonics that represent solutions of the Maxwell equations. We use these modes to express the equations for the T-matrix elements in terms of computationally tractable coefficient functions.

physics.optics↗

arXiv2001

Spatial reorientation of azobenzene side groups of a liquid crystalline polymer induced with linearly polarized light

The photoinduced 3D orientational structures in the films of liquid crystalline polyester, containing azobenzene side groups, are studied both experimentally and theoretically. By using the null ellipsometry and the UV absorption methods, preferential in-plane alignment of the azobenzene fragments and in-plane reorientation under irradiation with polarized UV light are established. The uniaxial and biaxial orientational configurations of the azobenzene chromophores are detected. The biaxiality is observed in the intermediate stages of irradiation, whereas the uniaxial structure is maintained in the photosaturated state. The components of the order parameter tensor of the azobenzene fragments are estimated for the initial state and after different doses of irradiation. The proposed theory takes into account biaxiality of the induced structures. Numerical analysis of the master equations for the order parameter tensor is found to yield the results that are in good agreement with the experimental dependencies of the order parameter components on the illumination time.

cond-mat.mtrl-sci↗