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The probability per unit time for a thermally activated Brownian particle to escape over a potential well is, in general, well-described by Kramers's theory. Kramers showed that the escape time decreases exponentially with increasing barrier height. The dynamics slow down when the particle is charged and subjected to a Lorentz force due to an external magnetic field. This is evident via a rescaling of the diffusion coefficient entering as a prefactor in the Kramers's escape rate without any impact on the barrier-height-dependent exponent. Here, we show that the barrier height can be effectively changed when the charged particle is subjected to a vortex flow. While the vortex alone does not affect the mean escape time of the particle, when combined with a magnetic field, it effectively pushes the fluctuating particle either radially outside or inside depending on its sign relative to that of the magnetic field. In particular, the effective potential over which the particle escapes can be changed to a flat, a stable, and an unstable potential by tuning the signs and magnitudes of the vortex and the applied magnetic field. Notably, the last case corresponds to enhanced escape dynamics. [ABSTRACT FROM AUTHOR]

The equilibrium structure and dynamics of a single polymer chain in a thermal solvent is by now wellunderstood in terms of scaling laws. Here, we consider a polymer in a bacterial bath, i.e., in a solvent consisting of active particles which bring in nonequilibrium fluctuations. Using computer simulations of a self-avoiding polymer chain in two dimensions which is exposed to a dilute bath of active particles, we show that the Flory-scaling exponent is unaffected by the bath activity provided the chain is very long. Conversely, for shorter chains, there is a nontrivial coupling between the bacteria intruding into the chain which may stiffen and expand the chain in a nonuniversal way. As a function of the molecular weight, the swelling first scales faster than described by the Flory exponent, then an unusual plateau-like behaviour is reached and finally a crossover to the universal Flory behaviour is observed. As a function of bacterial activity, the chain end-to-end distance exhibits a pronounced non-monotonicity. Moreover, the mean-square displacement of the center of mass of the chain shows a ballistic behaviour at intermediate times as induced by the active solvent. Our predictions are verifiable in two-dimensional bacterial suspensions and for colloidal model chains exposed to artificial colloidal microswimmers. [ABSTRACT FROM AUTHOR]

The ionic composition and pair correlations in fluid phases of realistically salt-free charged colloidal sphere suspensions are calculated in the primitive model. We obtain the number densities of all ionic species in suspension, including low-molecular weight microions, and colloidal macroions with acidic surface groups, from a self-consistent solution of a coupled physicochemical set of nonlinear algebraic equations and non-mean-field liquid integral equations. Here, we study suspensions of colloidal spheres with sulfonate or silanol surface groups, suspended in demineralized water that is saturated with carbon dioxide under standard atmosphere. The only input required for our theoretical scheme are the acidic dissociation constants pKa, and effective sphere diameters of all involved ions. Our method allows for an ab initio calculation of colloidal bare and effective charges, at high numerical efficiency. [ABSTRACT FROM AUTHOR]

The distance-resolved effective interaction between two colloidal particles in a subcritical solvent is explored both by an explicit and implicit modeling. An implicit solvent approach based on a simple thermodynamic interface model is tested against grand-canonical Monte Carlo computer simulations using explicit Lennard-Jones solvent molecules. Close to liquid-gas coexistence, a joint gas bubble surrounding the colloidal particle pair yields an effective attraction between the colloidal particles, the strength of which can be vastly tuned by the solvophobicity of the colloids. The implicit model is in good agreement with our explicit computer simulations, thus enabling an efficient modeling and evaluation of colloidal interactions and self-assembly in subcritical solvent environments. [ABSTRACT FROM AUTHOR]

At equilibrium, colloidal particles in a subcritical liquid suspension are surrounded by a drying layer if the colloid has solvophobic interaction. Using Brownian dynamics computer simulations, we investigate the nonequilibrium response of this layer to a strong external driving force. We find that the driven colloidal particle dresses itself with more particles than in the equilibrium drying layer. The effective interaction between two such dressed particles exhibits a deep drive-induced attraction due to a stretched joint gas bubble. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of the structural properties and phase behavior of spherical, loosely cross-linked ionic microgels that possess a low monomer concentration. The analysis is based on the recently derived effective interaction potential between such particles [A. R. Denton, Phys. Rev. E 67, 011804 (2003)]. By employing standard tools from the theory of the liquid state, we quantitatively analyze the pair correlations in the fluid and find anomalous behavior above the overlap concentration, similar to the cases of star-branched neutral and charged polymers. We also employ an evolutionary algorithm in order to predict the crystalline phases of the system without any a priori assumptions regarding their symmetry class. A very rich phase diagram is obtained, featuring two reentrant melting transitions and a number of unusual crystal structures. At high densities, both the Hansen–Verlet freezing criterion [J.-P. Hansen and L. Verlet, Phys. Rev. 184, 151 (1969)] and the Lindemann melting criterion [F. A. Lindemann, Phys. Z. 11, 609 (1910)] lose their validity. The topology of the phase diagram is altered when the steric interactions between the polymer segments become strong enough, in which case the lower-density reentrant melting disappears and the region of stability of the fluid is split into two disconnected domains, separated by intervening fcc and bcc regions. [ABSTRACT FROM AUTHOR]

Complex formation between a single polyelectrolyte chain and ionic surfactant molecules is studied by computer simulation of a “primitive” electrolyte model with explicit charges. The surfactant head carries a charge opposite to that of the polyelectrolyte. The neutral flexible tail is modeled by tethered hard spheres. A molecular bottle-brush architecture of the resulting polyelectrolytesurfactant-complex is observed. The end-to-end distance of the polyelectrolyte is found to behave in a nonmonotonic fashion for increasing Coulomb coupling: it first gets stretched and then the stretching is reduced by self-assembling of surfactant molecules along the polyelectrolyte. The end-to-end distance of the polyelectrolyte in the complex is more pronounced for long surfactant tails. Upon addition of salt to the complex, ionic surfactant molecules condensed onto the polyelectrolyte are replaced by salt microions which leads to a weakening of the complex and to reduced end-to-end distance. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The influence of temperature on the vibron–phonon combination band in the Raman and infrared (ir) spectra of the N2 and O2 molecular crystals supports the possibility of an assignment of the main features in the sideband to strong contributions from translational and librational phonons in points of high symmetry in the reciprocal lattices. The temperature behavior in Raman and ir sideband spectra in α-N2 is attributed to distinct anharmonicities in the isotropic and anisotropic parts of the potential and to different coupling mechanisms, resulting in a librational and translational weighted one-phonon density of states (DOS). In contrast, such an interpretation is not feasible for the poorly structured CO sideband, although the crystal structures of CO and N2 are nearly identical. The difference is attributed to strong anharmonicities and the presence of a weak dipole moment in the former, which introduces strong lattice mode coupling. Crossing the α–β phase transition has marked effects on the sidebands (shape, intensity) in both N2 and O2 crystals, which reflects the orientational disorder in the β phase of the former and the importance of the change in magnetic interactions in the latter. For both molecular crystals, the vibron–phonon coupling to the electromagnetic field is stronger in the ir than in the Raman spectra and seems predominantly due to electrical anharmonicities. [ABSTRACT FROM AUTHOR]

We study an exactly soluble model for planar interfacial kinetics for both locally conserved and nonconserved order parameters, where the Landau–Ginzburg potential is approximated by two parabolas. Long-range interactions with arbitrary interaction kernels are included. In particular, the relaxation of the initial order parameter interfacial profile to its equilibrium profile is considered. Furthermore, the steady-state solution of planar interfacial growth for a nonconserved order parameter is determined. Here, the influence of a second nonconserved order parameter is also discussed. [ABSTRACT FROM AUTHOR]

FTIR investigations on polycrystalline CO2 at various temperatures show Stokes and anti-Stokes sidebands coupled to the vibrational modes v3, v2, and v+/v- (Fermi dyad). The latter is infrared forbidden and may be eliminated from spectra of good quality polycrystalline films. The main peaks in the structure of the sidebands may be assigned to phonons from points of highest symmetry in the Brillouin zone. The vibron–phonon coupling process is mode specific and driven by electrical anharmonicity in case of v+ and v-. For the infrared allowed fundamentals, and especially at the Γ point, the coupling process is also driven by mechanical anharmonicity. The dispersion of these internal modes at k=0 influences the coupled phonons and thus the one phonon density of states is correctly mirrored by the v+ and v- sidebands only. [ABSTRACT FROM AUTHOR]

Vibron-phonon excitation bands, phonon sidebands to the zero phonon line, mirror the one-phonon density of states g(ω) calculated in the harmonic approximation. The origin of bands in the sideband is investigated, e.g., phonon contributions from librational and translational modes or phonons at special points of the Brillouin zone. The temperature-dependent structure of the sideband (frequency shifts, line broadening) is due to anharmonic processes which modify g(ω): Temperature-dependent frequency shifts of maxima in the sideband are shown to depend mainly on the volume effect, whereas line broadening is due to phonon–phonon interactions, which are simulated in the lattice dynamics calculations of g(ω) by a special modeling procedure. Compared to the CO2 solid, effects are more pronounced in the N2 crystal due to the presence of strong mechanical and electrical anharmonicities. The latter give rise to multiphonon contributions in the vibron-phonon excitation process. Intensity changes with temperature can be explained by the thermal weighting of the one-phonon density of states. [ABSTRACT FROM AUTHOR]

To gain insight into the fundamental processes determining the motion of macromolecules in polymeric matrices, the dynamical hindrance of polymeric dextran molecules diffusing as probe through a polyacrylamide hydrogel is systematically explored. Three complementary experimental methods combined with Brownian dynamics simulations are used to study a broad range of dextran molecular weights and salt concentrations. While multi-parameter fluorescence image spectroscopy (MFIS) is applied to investigate the local diffusion of single molecules on a microscopic length scale inside the hydrogel, a macroscopic transmission imaging (MTI) fluorescence technique and nuclear magnetic resonance (NMR) are used to study the collective motion of dextrans on the macroscopic scale. These fundamentally different experimental methods, probing different length scales of the system, yield long-time diffusion coefficients for the dextran molecules which agree quantitatively. The measured diffusion coefficients decay markedly with increasing molecular weight of the dextran and fall onto a master curve. The observed trends of the hindrance factors are consistent with Brownian dynamics simulations. The simulations also allow us to estimate the mean pore size for the herein investigated experimental conditions. In addition to the diffusing molecules, MFIS detects temporarily trapped molecules inside the matrix with diffusion times above 10 ms, which is also confirmed by anisotropy analysis. The fraction of bound molecules depends on the ionic strength of the solution and the charge of the dye. Using fluorescence intensity analysis, also MTI confirms the observation of the interaction of dextrans with the hydrogel. Moreover, pixelwise analysis permits to show significant heterogeneity of the gel on the microscopic scale. [ABSTRACT FROM AUTHOR]

We consider two-dimensional Yukawa systems in a perpendicular magnetic field. Computer simulations of both one-component and binary systems are used to explore the equilibrium particle dynamics in the fluid state. The mobility is found to scale with the inverse of the magnetic field strength (Bohm diffusion), for strong fields (?c/?p≳1). For bidisperse mixtures, the magnetic field dependence of the long-time mobility depends on the particle species, providing an external control of their mobility ratio. At large magnetic fields, the highly charged particles are almost immobilized by the magnetic field and form a porous matrix of obstacles for the mobile low-charge particles. [ABSTRACT FROM AUTHOR]

Crystallization in a two-dimensional strongly coupled plasma from a rapidly cooled fluid is found to be efficiently blocked by an external magnetic field. Beyond a threshold of the magnetic field strength B, the relaxation time to the equilibrium crystal increases exponentially with B, which is attributed to an impeded conversion of potential to kinetic energy. Our finding is opposed to the standard picture of two-dimensional freezing of one-component systems which does not exhibit a nucleation barrier and opens the way to keep two-dimensional fluids metastable over long times. [ABSTRACT FROM AUTHOR]

We explore the crystallization in a colloidal monolayer on a structured template starting from a few-particle nucleus. The competition between the substrate structure and that of the growing crystal induces a new crystal growth scenario. Unlike with the crystal growth in the bulk where a welldefined and connected crystal-fluid interface grows into the fluid, we identify a mechanism where a 'compatibility wave' of the prescribed nucleus with the underlying substrate structure dictates the growth direction and efficiency. The growth process is strongly anisotropic and proceeds via transient island formation in front of an initial solid-fluid interface. We demonstrate the validity of this compatibility wave concept for a large class of substrate structures including a square lattice and a quasicrystalline pattern. Dynamical density functional theory that provides a microscopic approach to the crystallization process is employed for colloidal hard spheres. Our predictions can be verified in experiments on confined colloids and also bear consequences for molecular crystal growth on structured substrates. [ABSTRACT FROM AUTHOR]

Collective properties of many rodlike circle swimmers are explored by computer simulations in two spatial dimensions. In the model considered, the center of mass of a single swimmer moves on a circle with radius R. Therefore, the model provides an interpolation between an interacting self-propelled-rod model for linear swimmers (R → ∞) and that of interacting passive rotors (R = 0). We map out the state diagram for various swimmer densities and radii R. For increasing density, the dilute state is followed by vortices consisting of single particles (singlet-vortex state), where neighboring particles are perpendicularly oriented, and vortices of swimmer pairs (doublet-vortex state). The vortices exhibit strong structural ordering on an array. At higher densities, a slowed rotor fluid with a significant degree of mutual rotation hindrance occurs. The single-particle vortex structure becomes unstable above a threshold in the circling radius R, while pair vortices are stable only for intermediate radii R. A simple theory is proposed to predict the topology of the state diagram. Our results are verifiable for bacterial and artificial rodlike circle swimmers. [ABSTRACT FROM AUTHOR]

Strongly coupled plasmas in which the interaction energy exceeds the kinetic energy play an important role in many astrophysical and laboratory systems including compact stars, laser plasmas and dusty plasmas. They exhibit many unusual collective properties, such as liquid or crystalline behaviour, peculiar oscillation spectra and transport properties. Recently, strongly coupled plasmas were studied in the presence of a strong magnetic field by computer simulations, and strong modifications of their transport properties and oscillation spectra were observed. While strong magnetization is common in stellar systems it is practically impossible to achieve in complex plasmas due to the large mass of the dust particles. Here we discuss a recently demonstrated approach to achieve very strong 'magnetization' by a rotation of the neutral gas, and we present new results for macroscopic two-dimensional systems. [ABSTRACT FROM AUTHOR]

The director field in a plastic crystal is calculated by particle-resolved Monte Carlo computer simulations of two-dimensional, slightly anisometric hard spherocylinders exposed to an external periodic substrate potential. We investigate the structure of the director field in the Wigner-Seitz cell and find a topological defect structure that can be controlled with the substrate potential. At zero potential we find a charge -1/2 defect at the corners besides the expected defect in the centre with unit topological charge. When switching the substrate potential on, the corner defects are surrounded by three satellite defects which bear charge -1/2, too. Additionally, we then find two charge +1/2 defects on each edge of the unit cell. Finally, within a simplified model, we obtain a qualitative explanation for this defect structure. Our predictions can in principle be verified by using particle-resolved experiments of colloidal plastic crystals. [ABSTRACT FROM AUTHOR]

A novel and flexible experiment is reported for investigation of the non-equilibrium melting behaviour of model crystals made from charged colloidal spheres. In a slit geometry, polycrystalline material formed in a low salt region is driven by hydrostatic pressure up an evolving gradient in salt concentration and melts at large salt concentration. Depending on particle and initial salt concentration, driving velocity and the local salt concentration, complex morphologic evolution is observed. Crystal-melt interface positions and the melting velocity are obtained quantitatively from time-resolved Bragg and polarisation microscopic measurements. A simple theoretical model predicts the interface to first advance, then for balanced drift and melting velocities to become stationary at a salt concentration larger than the equilibrium melting concentration. It also describes the relaxation of the interface to its equilibrium position in a stationary gradient after stopping the drive in different manners. The influence of the gradient strength on the resulting interface morphology and a shear-induced morphologic transition from polycrystalline to oriented single crystalline material before melting are discussed. [ABSTRACT FROM AUTHOR]

The long-standing observations that different amorphous materials exhibit a pronounced enhancement of viscosity and eventually vitrify on compression or cooling continue to fascinate and challenge scientists, on the ground of their physical origin and practical implications. Glass formation is a generic phenomenon, observed in physically quite distinct systems that encompass hard and soft particles. It is believed that a common underlying scenario, namely cage formation, drives dynamical arrest, especially at high concentrations. Here, we identify a novel, asymmetric glassy state in soft colloidal mixtures, which is characterized by strongly anisotropically distorted cages, bearing similarities to those of hard-sphere glasses under shear. The anisotropy is induced by the presence of soft additives. This phenomenon seems to be generic to soft colloids and its origins lie in the penetrability of the constituent particles. The resulting phase diagram for mixtures of soft particles is clearly distinct from that of hard-sphere mixtures and brings forward a rich variety of vitrified states that delineate an ergodic lake in the parameter space spanned by the size ratio between the two components and by the concentration of the additives. Thus, a new route opens for the rational design of soft particles with desired tunable rheological properties. [ABSTRACT FROM AUTHOR]

The aim of this paper is to review and to preview some selected topics of crystal nucleation in colloidal suspensions. First we discuss how the structure of critical nuclei can be calculated by computer simulations, in particular how linear shear flow affects the size and shape of the critical nuclei. Second, we preview the possibilities to access heterogeneous crystal nucleation and dynamics of a crystal by using the recently developed formalism of dynamical density functional theory. In particular, data for global crystal heating are presented. [ABSTRACT FROM AUTHOR]

We employ dynamical density functional theory (DDFT) and Brownian Dynamics (BD) simulations to examine the fully developed dynamics of ultrasoft colloids interacting via a Gaussian pair potential in time-dependent external fields. The DDFT formalism employed is that of Marconi and Tarazona [ J. Chem. Phys ., 110, 8032 (1999)], which allows for determination of the time-dependent density profile based on knowledge of the static, equilibrium density functional. Three different dynamical situations are examined: firstly, the behaviour of Gaussian particles in a spherical cavity of oscillating size, including both sudden and continuous changes in the size of the cavity. Secondly, a spherical cavity with a fixed size but varying sharpness. Finally, to investigate a strong inhomogeneity in the density profile we study the diffusion of one layer of particles which is initially strongly confined and separated from the remaining system via an external potential. In all cases, DDFT is in excellent agreement with BD results, demonstrating the applicability of the theory to dynamical problems involving overdamped interacting particles in a solvent. [ABSTRACT FROM AUTHOR]