One nanosecond photon correlation spectroscopy on smectic liquid crystal films

Analytical model of critical buckling transition for smectic liquid crystal based on the viscoelastic scaling of coarse-grained molecular dynamics

The buckling transition of smectic liquid crystals (LCs) is important not only as fundamental physics but also for the rational design of devices to make use of their optical and mechanical properties. However, there exists a huge gap between the specific knowledge and universal analytical formulation. We have conducted coarse-grained molecular dynamics (CGMD) simulations with the force field optimized for the description of buckling phenomena including topological defects to link the molecular nature and continuum formulation. The simulations reveal the viscoelastic characteristics where the critical strain and the compression modulus highly depend on the strain rate as well as the number of layers. Therefore, we formulate the scaling model whose coupling constants depend on both strain rate and domain size. The model reproduces the CGMD results as well as experimental and theoretical values in existing literature. Furthermore, we elucidate from this model that the critical buckling behavior is determined by the competition between the suppression of compression-induced flow and the undulation fluctuation of layers. The framework consisting of the CGMD simulation and the scaling model enables us to estimate the buckling characteristics of smectic LCs reflecting their molecular structures in a wide range from the low-frequency regime that can be verified by experiments to the high-frequency regime beyond the reach of it.

Derivation of coarse-grained force fields for buckling-induced topological defects of liquid crystals

Microscopic details of buckling-induced topological defects are required for molecular design of smectic liquid crystals to control buckling instability of the layers. In this study, we present a multiobjective optimization method to derive the coarse-grained (CG) force fields with sufficiently precise buckling characteristics including the molecular details for molecular dynamics (MD) simulations. We perform CGMD simulations of buckling deformation at sample points in the CG force field parameter space, from which the response surfaces of objective functions such as the scalar orientational order parameters, critical angles of layer collapse, and radial distribution functions are estimated. Since not all objective functions can be optimized simultaneously, we use a genetic algorithm to calculate the Pareto set of optimal solutions. We select the models with different molecular head-tail symmetries to study buckling deformation. The extracted CG model successfully reproduces the buckling deformation in terms of the collapse of smectic layers through the generation of dislocations with dipole disclinations. We also find that the molecular symmetry is a dominant factor to control the class of the buckling-induced dislocations.

Light scattering by liquid surfaces, new developments

The surface light scattering technique is presented, highlighting recent technical improvements and describing studies of various types of surfaces. The technique is non-invasive, but delicate to handle and no commercial instruments are available yet. The technique gives however interesting information difficult to obtain otherwise, for instance on out-of-equilibrium surfaces, surfaces of very low tension, or systems close to solidification. Many studies were performed with monolayers of surface-active molecules at the surface of water. In this case, surface viscoelastic parameters can be determined at high frequencies (10 kHz- 1 MHz), complementing usefully the data obtained at lower frequencies with other techniques. As with these other techniques, inconsistencies such as negative surface viscosities are sometimes reported. The origin of these anomalies is not yet fully clarified. The problem deserves further work, in order to achieve a satisfactory description of the motion of surfactant or polymer-laden surfaces.

Surface effects on the relaxation dynamics of hexatic-B liquid-crystal films

Based in a linearized hydrodynamic model, we study the relaxation dynamics of hexatic-B liquid-crystal films in two distinct cases. First, we investigate the hexatic order relaxing through a purely diffusive process for which the particles velocity field is assumed to be negligible. In this case, the asymptotic relaxation of deformations in the hexatic order presents a pronounced dependence on the boundary conditions. We found that a surface tilted order is enough to drive the slowest relaxation mode from an acoustic to an optic character. Second, we study the viscous case which is characterized by the coupling between the particles velocity field and the bond orientational order. In this case, underdamped modes with oscillatory-exponential relaxation are obtained on a narrow range of wave vectors. Further, the viscous relaxation of surface and inner layers deformations display distinct scaling behaviors under a surface tilted order.