123 research outputs found
Electrophoresis of colloidal dispersions in the low-salt regime
We study the electrophoretic mobility of spherical charged colloids in a
low-salt suspension as a function of the colloidal concentration. Using an
effective particle charge and a reduced screening parameter, we map the data
for systems with different particle charges and sizes, including numerical
simulation data with full electrostatics and hydrodynamics and experimental
data for latex dispersions, on a single master curve. We observe two different
volume fraction-dependent regimes for the electrophoretic mobility that can be
explained in terms of the static properties of the ionic double layer.Comment: Substantially revised versio
Effective Interactions and Volume Energies in Charge-Stabilized Colloidal Suspensions
Charge-stabilized colloidal suspensions can be conveniently described by
formally reducing the macroion-microion mixture to an equivalent one-component
system of pseudo-particles. Within this scheme, the utility of a linear
response approximation for deriving effective interparticle interactions has
been demonstrated [M. J. Grimson and M. Silbert, Mol. Phys. 74, 397 (1991)].
Here the response approach is extended to suspensions of finite-sized macroions
and used to derive explicit expressions for (1) an effective electrostatic pair
interaction between pseudo-macroions and (2) an associated volume energy that
contributes to the total free energy. The derivation recovers precisely the
form of the DLVO screened-Coulomb effective pair interaction for spherical
macroions and makes manifest the important influence of the volume energy on
thermodynamic properties of deionized suspensions. Excluded volume corrections
are implicitly incorporated through a natural modification of the inverse
screening length. By including nonlinear response of counterions to macroions,
the theory may be generalized to systematically investigate effective many-body
interactions.Comment: 13 pages (J. Phys.: Condensed Matter, in press
Simulation of Claylike Colloids
We investigate properties of dense suspensions and sediments of small
spherical silt particles by means of a combined Molecular Dynamics (MD) and
Stochastic Rotation Dynamics (SRD) simulation. We include van der Waals and
effective electrostatic interactions between the colloidal particles, as well
as Brownian motion and hydrodynamic interactions which are calculated in the
SRD-part. We present the simulation technique and first results. We have
measured velocity distributions, diffusion coefficients, sedimentation
velocity, spatial correlation functions and we have explored the phase diagram
depending on the parameters of the potentials and on the volume fraction.Comment: 20 pages, 14 figure
Simulation of a two-dimensional model for colloids in a uniaxial electric field
We perform Monte Carlo simulations of a simplified two-dimensional model for
colloidal hard spheres in an external uniaxial AC electric field.
Experimentally, the external field induces dipole moments in the colloidal
particles, which in turn form chains. We therefore approximate the system as
composed of well formed chains of dipolar hard spheres of a uniform length. The
dipolar interaction between colloidal spheres gives rise to an effective
interaction between the chains, which we treat as disks in a plane, that
includes a short range attraction and long range repulsion. Hence, the system
favors finite clustering over bulk phase separation and indeed we observe at
low temperature and density that the system does form a cluster phase. As
density increases, percolation is accompanied by a pressure anomaly. The
percolated phase, despite being composed of connected, locally crystalline
domains, does not bear the typical signatures of a hexatic phase. At very low
densities, we find no indication of a "void phase" with a cellular structure
seen recently in experiments.Comment: 10 pages, 14 figure
Shear Viscosity of Clay-like Colloids in Computer Simulations and Experiments
Dense suspensions of small strongly interacting particles are complex
systems, which are rarely understood on the microscopic level. We investigate
properties of dense suspensions and sediments of small spherical Al_2O_3
particles in a shear cell by means of a combined Molecular Dynamics (MD) and
Stochastic Rotation Dynamics (SRD) simulation. We study structuring effects and
the dependence of the suspension's viscosity on the shear rate and shear
thinning for systems of varying salt concentration and pH value. To show the
agreement of our results to experimental data, the relation between bulk pH
value and surface charge of spherical colloidal particles is modeled by
Debye-Hueckel theory in conjunction with a 2pK charge regulation model.Comment: 15 pages, 8 figure
Effect of aggregation on thermal conduction in colloidal nanofluids
Using effective medium theory we demonstrate that the thermal conductivity of nanofluids can be significantly enhanced by the aggregation of nanoparticles into clusters. The enhancement is based purely on conduction and does not require a novel mechanism. Predictions of the effective medium theory are in excellent agreement with detailed numerical calculations on model nanofluids involving fractal clusters and show the importance of cluster morphology on thermal conductivity enhancements
Effective Interactions and Volume Energies in Charged Colloids: Linear Response Theory
Interparticle interactions in charge-stabilized colloidal suspensions, of
arbitrary salt concentration, are described at the level of effective
interactions in an equivalent one-component system. Integrating out from the
partition function the degrees of freedom of all microions, and assuming linear
response to the macroion charges, general expressions are obtained for both an
effective electrostatic pair interaction and an associated microion volume
energy. For macroions with hard-sphere cores, the effective interaction is of
the DLVO screened-Coulomb form, but with a modified screening constant that
incorporates excluded volume effects. The volume energy -- a natural
consequence of the one-component reduction -- contributes to the total free
energy and can significantly influence thermodynamic properties in the limit of
low-salt concentration. As illustrations, the osmotic pressure and bulk modulus
are computed and compared with recent experimental measurements for deionized
suspensions. For macroions of sufficient charge and concentration, it is shown
that the counterions can act to soften or destabilize colloidal crystals.Comment: 14 pages, including 3 figure
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
Site directed biotinylation of filamentous phage structural proteins
Filamentous bacteriophages have been used in numerous applications for the display of antibodies and random peptide libraries. Here we describe the introduction of a 13 amino acid sequence LASIFEAQKIEWR (designated BT, which is biotinylated in vivo by E. coli) into the N termini of four of the five structural proteins of the filamentous bacteriophage fd (Proteins 3, 7, 8 and 9). The in vivo and in vitro biotinylation of the various phages were compared. The production of multifunctional phages and their application as affinity reagents are demonstrated
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