Search

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]