Ab initio molecular dynamics of liquid carbon disulphide

An ab initio molecular dynamics study has been made of molecular liquid CS₂ using a density functional approach in the quasi-local ‘generalized gradient approach’ (GGA). Various aspects of the liquid at a microscopic level have been investigated including dynamic effects on structure such as velocity autocorrelation functions, vibrational modes and position and angular correlation functions. Results are presented on the molecular electronic structure at finite temperature, including the fluctuating molecular dipole moment and its relationship to atomic position and normal modes. Although the GGA is explicitly incapable of describing non-local exchange-correlation effects which are conventionally believed to be important in this system, these results are nevertheless in good agreement with experiment.     

Structure of liquid bismuth calculated from pseudo-potentials and molecular dynamics

Earlier calculations on liquid bismuth have been done by using five conduction electrons. However, this metal presents, in the liquid state, a gap in its electronic density of states that clearly separates the s and the p bands. Thus, the number of free electrons to be considered is affected by the existence of the gap. Previous calculations on the structure of liquid bismuth with five conduction electrons did not give satisfactory results. Molecular dynamics calculations using an effective potential derived from the empty core potential (ECP) and from the local optimised model potential (OMP) are presented and compared with the results obtained with three (p) and five (s + p) conduction electrons. The results, obtained for the first time to our knowledge for liquid bismuth at different temperatures, with three conduction electrons and OMP are in very good agreement with the Waseda structural experimental data. This approach using three conduction electrons is confirmed by resistivity calculations using the t-matrix model.     

Structure and dynamics of the Frenkel-Kontorova dislocations in electroconvection in liquid crystals

The Frenkel-Kontorova instability is studied in a 1D lattice of domains formed during electroconvection in nematic liquid crystals twisted by π/2. It is found that generation of defects by such instability can be observed in this model medium. Among other things, it is shown that several types of defects with singular and nonsingular cores, as well as with a extended core, are formed in the 1D domain structure above the electroconvective instability threshold. The extended cores of dislocations are dissociated into a line, and the entire structure is isomorphic to two partial dislocations spaced by a certain distance, which are not observed in free form. Defects with a nonsingular core (zero topological index) exist owing to spiral hydrodynamic flows in convective rolls and are not observed in layers with a homogeneous orientation of molecules. It is shown that the formation of both types of defects follows the scenario of decay of dislocations with extended cores via detachment of nonsingular defects (i.e., discretely); as a result, a dislocation with a singular core is left. “Breather” defects, which are the result of periodic creation and annihilation of dislocations with a topological index of ±1, are also observed. The effect of defects on the transition from the 1D to 2D structures is considered.     

Structure and dynamics of liquid crystalline pattern formation in drying droplets of DNA

We investigate the formation of ringlike deposits in drying drops of DNA. In analogy with the colloidal "coffee rings," DNA is transported to the perimeter by the capillary flow. At the droplet edge, however, DNA forms a lyotropic liquid crystal (LC) with concentric chain orientations to minimize the LC elastic energy. During the final stages of drying, the contact line retracts, and the radial stress causes undulations at the rim that propagate inward through the LC and form a periodic zigzag structure. We examine the phenomenon in terms of a simple model based on LC elasticity.     

Structure and dynamics of liquid formamide at high pressure and high temperature: comparison of X-ray diffraction and molecular dynamics results

New molecular dynamics simulations with optimised potentials for liquid simulation are presented for liquid formamide at high pressures and high temperatures. The structural results are compared to those found by X-ray diffraction measurements, and the H-bonding structure is analysed in detail. While it is ambiguous from purely experimental data, the simulation results support the idea that pressure increase can enhance ring dimer formation at the expense of linear chain structure, but increasing temperature results in an opposite effect. Dynamical results (reorientational correlation times and lifetimes of hydrogen bonds) are in agreement with these findings.     

Structure and elastic properties of a nematic liquid crystal: A theoretical treatment and molecular dynamics simulation

The Frank elasticity constants which describe splay (K ₁), twist (K ₂), and bend (K ₃) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations rest on statistical-mechanical approaches where the absolute values of K _i (i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants were employed based on different level of approximation about the orientational order and molecular correlations. The best agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution is observed for short intermolecular distances. Received 22 March 2000 and Received in final form 9 June 2000     

Structure and dynamics of cellular systems

This review addresses recent progress in the analysis and modelling of disordered cell structures. It focuses on planar systems, for which most research has been performed. The subject is approached from a general viewpoint rather than focusing on the specific evolution of certain systems. To this end, results of studies performed in completely different disciplines, ranging from applied sciences, biology and physics to mathematics, are gathered and discussed systematically. Special emphasis is laid on common properties of the different structures, including a critical discussion of the information contained in typically measured quantities. Efficient techniques for the simulation of typical disordered cell structures are summarized, and novel theoretical approaches to the modelling of dynamical cell structures are presented. Here, special attention is paid to the application of the methods of statistical mechanics and the resulting implications. In several systems, a breakdown of order induced by variation of an external control parameter can be observed. Its diagnostics in Voronoi tessellations is presented focusing on the connection with the underlying physics. The discussion of planar cell structures is concluded with a presentation of the peculiar properties of exceptional systems, characterized by the coexistence of two different length scales or by scale invariance. This review concludes with a presentation of recent developments in the research on three-dimensional cell structures.     

Structure and dynamics of fractal aerogels

Silica aerogels form mutually self-similar fractal structures for a broad range of densities and preparation conditions. This allows to scale dynamical results obtained on longitudinal acoustic waves up to the onset of the phonon to fracton crossover. An effective value of the fracton dimension is found, and various possible interpretations of that value are discussed.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Interface dynamics in liquid crystals

We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns which have been both experimentally and theoretically pointed out. Received 5 August 1999 and Received in final form 13 September 1999     

Microscopic dynamics in liquid rubidium

The dynamical properties of a system simulating liquid rubidium are investigated by computer experiments. The data for several correlation functions are then compared with the results of a previous theory; particular emphasis is given to those processes probing the dynamics of the momentum transfer from an atom to its neighbours.     

Instability and dynamics of thin viscoelastic liquid films

The instability, rupture, and subsequent growth of holes in a thin Jeffreys-type viscoelastic film under the influence of long-range van der Waals force are investigated using both linear stability analysis and nonlinear numerical solutions. The linear stability analysis of full governing equations valid for arbitrary wave numbers shows that although fluid rheology does not influence the dominant length scale of the instability, it significantly affects the growth rate. It is shown that neglect of inertia and solvent dynamics results in a nonphysical singularity in the growth rate beyond a critical value of relaxation time. We further carry out numerical simulations of a set of long-wave, nonlinear differential equations (also derived in Rauscher et al., Eur. Phys. J. E 17, 373 (2005)) governing the evolution of the free surface. The nonlinear simulations, in their domain of validity, confirm the results of the linear analysis. Interestingly, results from nonlinear simulations further show that both for Newtonian and viscoelastic liquids, the shape and the dewetting dynamics of a hole are identical when examined in terms of a rescaled time which depends on rheological parameters. Thus, viscoelasticity of Jeffreys type merely accelerates the growth rate, without however affecting the important morphological characteristics.