267 research outputs found
Colloidal glass transition: Beyond mode-coupling theory
A new theory for dynamics of concentrated colloidal suspensions and the
colloidal glass transition is proposed. The starting point is the memory
function representation of the density correlation function. The memory
function can be expressed in terms of a time-dependent pair-density correlation
function. An exact, formal equation of motion for this function is derived and
a factorization approximation is applied to its evolution operator. In this way
a closed set of equations for the density correlation function and the memory
function is obtained. The theory predicts an ergodicity breaking transition
similar to that predicted by the mode-coupling theory, but at a higher density.Comment: to be published in PR
A simple patchy colloid model for the phase behavior of lysozyme dispersions
We propose a minimal model for spherical proteins with aeolotopic pair
interactions to describe the equilibrium phase behavior of lysozyme. The
repulsive screened Coulomb interactions between the particles are taken into
account assuming that the net charges are smeared out homogeneously over the
spherical protein surfaces. We incorporate attractive surface patches, with the
interactions between patches on different spheres modeled by an attractive
Yukawa potential. The parameters entering the attractive Yukawa potential part
are determined using information on the experimentally accessed gas-liquid-like
critical point. The Helmholtz free energy of the fluid and solid phases is
calculated using second-order thermodynamic perturbation theory. Our
predictions for the solubility curve are in fair agreement with experimental
data. In addition, we present new experimental data for the gas-liquid
coexistence curves at various salt concentrations and compare these with our
model calculations. In agreement with earlier findings, we observe that the
strength and the range of the attractive potential part only weakly depend on
the salt content
Enhanced structural correlations accelerate diffusion in charge-stabilized colloidal suspensions
Theoretical calculations for colloidal charge-stabilized and hard sphere
suspensions show that hydrodynamic interactions yield a qualitatively different
particle concentration dependence of the short-time self-diffusion coefficient.
The effect, however, is numerically small and hardly accessible by conventional
light scattering experiments. Applying multiple-scattering decorrelation
equipment and a careful data analysis we show that the theoretical prediction
for charged particles is in agreement with our experimental results from
aqueous polystyrene latex suspensions.Comment: 1 ps-file (MS-Word), 14 page
Self-diffusion coefficients of charged particles: Prediction of Nonlinear volume fraction dependence
We report on calculations of the translational and rotational short-time
self-diffusion coefficients and for suspensions of
charge-stabilized colloidal spheres. These diffusion coefficients are affected
by electrostatic forces and many-body hydrodynamic interactions (HI). Our
computations account for both two-body and three-body HI. For strongly charged
particles, we predict interesting nonlinear scaling relations and depending on volume fraction
, with essentially charge-independent parameters and . These
scaling relations are strikingly different from the corresponding results for
hard spheres. Our numerical results can be explained using a model of effective
hard spheres. Moreover, we perceptibly improve the known result for of
hard sphere suspensions.Comment: 8 pages, LaTeX, 3 Postscript figures included using eps
Gaussian density fluctuations, mode coupling theory, and all that
We consider a toy model for glassy dynamics of colloidal suspensions: a
single Brownian particle diffusing among immobile obstacles. If Gaussian
factorization of static density fluctuations is assumed, this model can be
solved without factorization approximation for any dynamic correlation
function. The solution differs from that obtained from the ideal mode coupling
theory (MCT). The latter is equivalent to including only some, positive
definite terms in an expression for the memory function. An approximate
re-summation of the complete expression suggests that, under the assumption of
Gaussian factorization of static fluctuations, mobile particle's motion is
always diffusive. In contrast, MCT predicts that the mobile particle becomes
localized at a high enough obstacle density. We discuss the implications of
these results for models for glassy dynamics.Comment: to be published in Europhys. Let
A corresponding states approach to Small-Angle-Scattering for polydisperse ionic colloidal fluids
Approximate scattering functions for polydisperse ionic colloidal fluids are
obtained by a corresponding states approach. This assumes that all pair
correlation functions of a polydisperse fluid are
conformal to those of an appropriate monodisperse binary fluid (reference
system) and can be generated from them by scaling transformations. The
correspondence law extends to ionic fluids a {\it scaling approximation} (SA)
successfully proposed for nonionic colloids in a recent paper. For the
primitive model of charged hard spheres in a continuum solvent, the partial
structure factors of the monodisperse binary reference system are evaluated by
solving the Orstein-Zernike (OZ) integral equations coupled with an approximate
closure. The SA is first tested within the mean spherical approximation (MSA)
closure, which allows analytical solutions. The results are found in good
overall agreement with exact MSA predictions up to relevant polidispersity. The
SA is shown to be an improvement over the ``decoupling approximation'' extended
to the ionic case. The simplicity of the SA scheme allows its application also
when the OZ equations can be solved only numerically. An example is then given
by using the hypernetted chain (HNC) closure. Shortcomings of the SA approach,
its possible use in the analysis of experimental scattering data and other
related points are also briefly addressed.Comment: 29 pages, 7 postscript figures (included), Latex 3.0, uses aps.sty,
to appear in Phys. Rev. E (1999
Aspects of the dynamics of colloidal suspensions: Further results of the mode-coupling theory of structural relaxation
Results of the idealized mode-coupling theory for the structural relaxation
in suspensions of hard-sphere colloidal particles are presented and discussed
with regard to recent light scattering experiments. The structural relaxation
becomes non-diffusive for long times, contrary to the expectation based on the
de Gennes narrowing concept. A semi-quantitative connection of the wave vector
dependences of the relaxation times and amplitudes of the final
-relaxation explains the approximate scaling observed by Segr{\`e} and
Pusey [Phys. Rev. Lett. {\bf 77}, 771 (1996)]. Asymptotic expansions lead to a
qualitative understanding of density dependences in generalized Stokes-Einstein
relations. This relation is also generalized to non-zero frequencies thereby
yielding support for a reasoning by Mason and Weitz [Phys. Rev. Lett {\bf 74},
1250 (1995)]. The dynamics transient to the structural relaxation is discussed
with models incorporating short-time diffusion and hydrodynamic interactions
for short times.Comment: 11 pages, 9 figures; to be published in Phys. Rev.
Short-time Rheology and Diffusion in Suspensions of Yukawa-type Colloidal Particles
A comprehensive study is presented on the short-time dynamics in suspensions
of charged colloidal spheres. The explored parameter space covers the major
part of the fluid-state regime, with colloid concentrations extending up to the
freezing transition. The particles are assumed to interact directly by a
hard-core plus screened Coulomb potential, and indirectly by solvent-mediated
hydrodynamic interactions (HIs). By comparison with accurate accelerated
Stokesian Dynamics (ASD) simulations of the hydrodynamic function H(q), and the
high-frequency viscosity, we investigate the accuracy of two fast and
easy-to-implement analytical schemes. The first scheme, referred to as the
pairwise additive (PA) scheme, uses exact two-body hydrodynamic mobility
tensors. It is in good agreement with the ASD simulations of H(q) and the
high-frequency viscosity, for smaller volume fractions up to about 10% and 20%,
respectively. The second scheme is a hybrid method combining the virtues of the
\delta\gamma-scheme by Beenakker and Mazur with those of the PA scheme. It
leads to predictions in good agreement with the simulation data, for all
considered concentrations, combining thus precision with computational
efficiency. The hybrid method is used to test the accuracy of a generalized
Stokes-Einstein (GSE) relation proposed by Kholodenko and Douglas, showing its
severe violation in low salinity systems. For hard spheres, however, this GSE
relation applies decently well
Energetics of the primary electron transfer reaction revealed by ultrafast spectroscopy on modified bacterial reaction centers
The modification of reaction centers from Rhodobacter sphaeroides by the introduction of pheophytins instead of bacteriopheophytins leads to interesting changes in the primary photosynthetic reaction: long-living populations of the excited electronic state of the special pair P* and the bacteriochlorophyll anion BâA show up. The data allow the determination of the energetics in the reaction center. The free energy of the first intermediate P+BâA, where the electron has reached the accessory bacteriochlorophyll BA lies â 450 cmâ1 below the initially excited special pair P*
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