143 research outputs found
Independent Ion Migration in Suspensions of Strongly Interacting Charged Colloidal Spheres
We report on sytematic measurements of the low frequency conductivity in
aequous supensions of highly charged colloidal spheres. System preparation in a
closed tubing system results in precisely controlled number densities between
1E16/m3 and 1E19/m^3 (packing fractions between 1E-7 and 1E-2) and electrolyte
concentrations between 1E-7 and 1E-3 mol/l. Due to long ranged Coulomb
repulsion some of the systems show a pronounced fluid or crystalline order.
Under deionized conditions we find s to depend linearily on the packing
fraction with no detectable influence of the phase transitions. Further at
constant packing fraction s increases sublinearily with increasing number of
dissociable surface groups N. As a function of c the conductivity shows
pronounced differences depending on the kind of electrolyte used. We propose a
simple yet powerful model based on independent migration of all species present
and additivity of the respective conductivity contributions. It takes account
of small ion macro-ion interactions in terms of an effectivly transported
charge. The model successfully describes our qualitatively complex experimental
observations. It further facilitates quantitative estimates of conductivity
over a wide range of particle and experimental parameters.Comment: 32 pages, 17 figures, 2 tables, Accepted by Physical Review
Non-equilibrium melting of colloidal crystals in confinement
We report on a novel and flexible experiment to investigate 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
polarization 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. We further discuss 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
Reentrant glass transition in a colloid-polymer mixture with depletion attractions
Performing light scattering experiments we show that introducing short-ranged
attraction to a colloidal suspension of nearly hard spheres by addition of free
polymer produces new glass transition phenomena. We observe a dramatic
acceleration of the density fluctuations amounting to the melting of a
colloidal glass. Increasing the strength of the attractions the system freezes
into another nonergodic state sharing some qualitative features with gel states
occurring at lower colloid packing fractions. This reentrant glass transition
is in qualitative agreement with recent theoretical predictions.Comment: 14 pages, 3 figure
Confined colloidal crystals in and out of equilibrium
Recent studies on confined crystals of charged colloidal particles are
reviewed, both in equilibrium and out of equilibrium. We focus in particular on
direct comparisons of experiments (light scattering and microscopy) with
lattice sum calculations and computer simulations. In equilibrium we address
buckling and crystalline multilayering of charged systems in hard and soft slit
confinement. We discuss also recent crystalline structures obtained for charged
mixtures. Moreover, we put forward possibilities to apply external
perturbations, in order to drive the system out of equilibrium. These include
electrolyte gradients as well as the application of shear and electric fields.Comment: Review article, 18 pages, 5 figure
Numerical electrokinetics
A new lattice method is presented in order to efficiently solve the
electrokinetic equations, which describe the structure and dynamics of the
charge cloud and the flow field surrounding a single charged colloidal sphere,
or a fixed array of such objects. We focus on calculating the electrophoretic
mobility in the limit of small driving field, and systematically linearise the
equations with respect to the latter. This gives rise to several subproblems,
each of which is solved by a specialised numerical algorithm. For the total
problem we combine these solvers in an iterative procedure. Applying this
method, we study the effect of the screening mechanism (salt screening vs.
counterion screening) on the electrophoretic mobility, and find a weak
non-trivial dependence, as expected from scaling theory. Furthermore, we find
that the orientation of the charge cloud (i. e. its dipole moment) depends on
the value of the colloid charge, as a result of a competition between
electrostatic and hydrodynamic effects.Comment: accepted for publication in Journal of Physics Condensed Matter
(proceedings of the 2012 CODEF conference
Colloidal electrophoresis: Scaling analysis, Green-Kubo relation, and numerical results
We consider electrophoresis of a single charged colloidal particle in a
finite box with periodic boundary conditions, where added counterions and salt
ions ensure charge neutrality. A systematic rescaling of the electrokinetic
equations allows us to identify a minimum set of suitable dimensionless
parameters, which, within this theoretical framework, determine the reduced
electrophoretic mobility. It turns out that the salt-free case can, on the Mean
Field level, be described in terms of just three parameters. A fourth
parameter, which had previously been identified on the basis of straightforward
dimensional analysis, can only be important beyond Mean Field. More complicated
behavior is expected to arise when further ionic species are added. However,
for a certain parameter regime, we can demonstrate that the salt-free case can
be mapped onto a corresponding system containing additional salt. The
Green-Kubo formula for the electrophoretic mobility is derived, and its
usefulness demonstrated by simulation data. Finally, we report on
finite-element solutions of the electrokinetic equations, using the commercial
software package COMSOL.Comment: To appear in Journal of Physics: Condensed Matter - special issue on
occasion of the CODEF 2008 conferenc
- …