1,467 research outputs found
Elastic Behavior of a Two-dimensional Crystal near Melting
Using positional data from video-microscopy we determine the elastic moduli
of two-dimensional colloidal crystals as a function of temperature. The moduli
are extracted from the wave-vector-dependent normal mode spring constants in
the limit and are compared to the renormalized Young's modulus of the
KTHNY theory. An essential element of this theory is the universal prediction
that Young's modulus must approach at the melting temperature. This is
indeed observed in our experiment.Comment: 4 pages, 3 figure
Many-body interactions and melting of colloidal crystals
We study the melting behavior of charged colloidal crystals, using a
simulation technique that combines a continuous mean-field Poisson-Boltzmann
description for the microscopic electrolyte ions with a Brownian-dynamics
simulation for the mesoscopic colloids. This technique ensures that many-body
interactions between the colloids are fully taken into account, and thus allows
us to investigate how many-body interactions affect the solid-liquid phase
behavior of charged colloids. Using the Lindemann criterion, we determine the
melting line in a phase-diagram spanned by the colloidal charge and the salt
concentration. We compare our results to predictions based on the established
description of colloidal suspensions in terms of pairwise additive Yukawa
potentials, and find good agreement at high-salt, but not at low-salt
concentration. Analyzing the effective pair-interaction between two colloids in
a crystalline environment, we demonstrate that the difference in the melting
behavior observed at low salt is due to many-body interactions
Testing the relevance of effective interaction potentials between highly charged colloids in suspension
Combining cell and Jellium model mean-field approaches, Monte Carlo together
with integral equation techniques, and finally more demanding many-colloid
mean-field computations, we investigate the thermodynamic behavior, pressure
and compressibility of highly charged colloidal dispersions, and at a more
microscopic level, the force distribution acting on the colloids. The
Kirkwood-Buff identity provides a useful probe to challenge the
self-consistency of an approximate effective screened Coulomb (Yukawa)
potential between colloids. Two effective parameter models are put to the test:
cell against renormalized Jellium models
Measuring the equation of state of a hard-disc fluid
We use video microscopy to study a two-dimensional (2D) model fluid of
charged colloidal particles suspended in water and compute the pressure from
the measured particle configurations. Direct experimental control over the
particle density by means of optical tweezers allows the precise measurement of
pressure as a function of density. We compare our data with theoretical
predictions for the equation of state, the pair-correlation function and the
compressibility of a hard-disc fluid and find good agreement, both for the
fluid and the solid phase. In particular the location of the transition point
agrees well with results from Monte Carlo simulations.Comment: 7 pages, to appear in EPL, slightly corrected versio
Effect of many-body interactions on the solid-liquid phase-behavior of charge-stabilized colloidal suspensions
The solid-liquid phase-diagram of charge-stabilized colloidal suspensions is
calculated using a technique that combines a continuous Poisson-Boltzmann
description for the microscopic electrolyte ions with a molecular-dynamics
simulation for the macroionic colloidal spheres. While correlations between the
microions are neglected in this approach, many-body interactions between the
colloids are fully included. The solid-liquid transition is determined at a
high colloid volume fraction where many-body interactions are expected to be
strong. With a view to the Derjaguin-Landau-Verwey-Overbeek theory predicting
that colloids interact via Yukawa pair-potentials, we compare our results with
the phase diagram of a simple Yukawa liquid. Good agreement is found at high
salt conditions, while at low ionic strength considerable deviations are
observed. By calculating effective colloid-colloid pair-interactions it is
demonstrated that these differences are due to many-body interactions. We
suggest a density-dependent pair-potential in the form of a truncated Yukawa
potential, and show that it offers a considerably improved description of the
solid-liquid phase-behavior of concentrated colloidal suspensions
Magnetic properties of Quantum Corrals from first principles calculations
We present calculations for electronic and magnetic properties of surface
states confined by a circular quantum corral built of magnetic adatoms (Fe) on
a Cu(111) surface. We show the oscillations of charge and magnetization
densities within the corral and the possibility of the appearance of
spin--polarized states. In order to classify the peaks in the calculated
density of states with orbital quantum numbers we analyzed the problem in terms
of a simple quantum mechanical circular well model. This model is also used to
estimate the behaviour of the magnetization and energy with respect to the
radius of the circular corral. The calculations are performed fully
relativistically using the embedding technique within the
Korringa-Kohn-Rostoker method.Comment: 14 pages, 9 figures, submitted to J. Phys. Cond. Matt. special issue
on 'Theory and Simulation of Nanostructures
Injection of photoelectrons into dense argon gas
The injection of photoelectrons in a gaseous or liquid sample is a widespread
technique to produce a cold plasma in a weakly--ionized system in order to
study the transport properties of electrons in a dense gas or liquid. We report
here the experimental results of photoelectron injection into dense argon gas
at the temperatureT=142.6 K as a function of the externally applied electric
field and gas density. We show that the experimental data can be interpreted in
terms of the so called Young-Bradbury model only if multiple scattering effects
due to the dense environment are taken into account when computing the
scattering properties and the energetics of the electrons.Comment: 18 pages, 10 figures, figure nr. 10 has been redrawn, to be submitted
to Plasma Sources Science and Technolog
On the nature of long-range contributions to pair interactions between charged colloids in two dimensions
We perform a detailed analysis of solutions of the inverse problem applied to
experimentally measured two-dimensional radial distribution functions for
highly charged latex dispersions. The experiments are carried out at high
colloidal densities and under low-salt conditions. At the highest studied
densities, the extracted effective pair potentials contain long-range
attractive part. At the same time, we find that for the best distribution
functions available the range of stability of the solutions is limited by the
nearest neighbour distance between the colloidal particles. Moreover, the
measured pair distribution functions can be explained by purely repulsive pair
potentials contained in the stable part of the solution.Comment: 6 pages, 5 figure
The osmotic pressure of charged colloidal suspensions: A unified approach to linearized Poisson-Boltzmann theory
We study theoretically the osmotic pressure of a suspension of charged
objects (e.g., colloids, polyelectrolytes, clay platelets, etc.) dialyzed
against an electrolyte solution using the cell model and linear
Poisson-Boltzmann (PB) theory. From the volume derivative of the grand
potential functional of linear theory we obtain two novel expressions for the
osmotic pressure in terms of the potential- or ion-profiles, neither of which
coincides with the expression known from nonlinear PB theory, namely, the
density of microions at the cell boundary. We show that the range of validity
of linearization depends strongly on the linearization point and proof that
expansion about the selfconsistently determined average potential is optimal in
several respects. For instance, screening inside the suspension is
automatically described by the actual ionic strength, resulting in the correct
asymptotics at high colloid concentration. Together with the analytical
solution of the linear PB equation for cell models of arbitrary dimension and
electrolyte composition explicit and very general formulas for the osmotic
pressure ensue. A comparison with nonlinear PB theory is provided. Our analysis
also shows that whether or not linear theory predicts a phase separation
depends crucially on the precise definition of the pressure, showing that an
improper choice could predict an artificial phase separation in systems as
important as DNA in physiological salt solution.Comment: 16 pages, 5 figures, REVTeX4 styl
On the fluid-fluid phase separation in charged-stabilized colloidal suspensions
We develop a thermodynamic description of particles held at a fixed surface
potential. This system is of particular interest in view of the continuing
controversy over the possibility of a fluid-fluid phase separation in aqueous
colloidal suspensions with monovalent counterions. The condition of fixed
surface potential allows in a natural way to account for the colloidal charge
renormalization. In a first approach, we assess the importance of the so called
``volume terms'', and find that in the absence of salt, charge renormalization
is sufficient to stabilize suspension against a fluid-fluid phase separation.
Presence of salt, on the other hand, is found to lead to an instability. A very
strong dependence on the approximations used, however, puts the reality of this
phase transition in a serious doubt. To further understand the nature of the
instability we next study a Jellium-like approximation, which does not lead to
a phase separation and produces a relatively accurate analytical equation of
state for a deionized suspensions of highly charged colloidal spheres. A
critical analysis of various theories of strongly asymmetric electrolytes is
presented to asses their reliability as compared to the Monte Carlo
simulations
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