25 research outputs found
Noise enhanced performance of ratchet cellular automata
We present the first experimental realization of a ratchet cellular automaton
(RCA) which has been recently suggested as an alternative approach for
performing logical operations with interacting (quasi) particles. Our study was
performed with interacting colloidal particles which serve as a model system
for other dissipative systems i.e. magnetic vortices on a superconductor or
ions in dissipative optical arrays. We demonstrate that noise can enhance the
efficiency of information transport in RCA and consequently enables their
optimal operation at finite temperatures.Comment: accepted for publication at Phys. Rew. Let
Charge ordering in quarter-filled ladder systems coupled to the lattice
We investigate charge ordering in the presence of electron-phonon coupling
for quarter-filled ladder systems by using Exact Diagonalization. As an example
we consider NaV2O5 using model parameters obtained from first-principles
band-structure calculations. The relevant Holstein coupling to the lattice
considerably reduces the critical value of the nearest-neighbor Coulomb
repulsion at which formation of the zig-zag charge-ordered state occurs, which
is then accompanied by a static lattice distortion. Energy and length of a
kink-like excitation on the background of the distorted lattice are calculated.
Spin and charge spectra on ladders with and without static distortion are
obtained, and the charge gap and the effective spin-spin exchange parameter J
are extracted. J agrees well with experimental results. Analysis of the
dynamical Holstein model, restricted to a small number of phonons, shows that
low frequency lattice vibrations increase the charge order, accompanied by
dynamically produced zig-zag lattice distortions.Comment: 11 pages, 17 figures, revised version as to appear in Phys. Rev.
A Stability Diagram for Dense Suspensions of Model Colloidal Al2O3-Particles in Shear Flow
In Al2O3 suspensions, depending on the experimental conditions very different
microstructures can be found, comprising fluid like suspensions, a repulsive
structure, and a clustered microstructure. For technical processing in
ceramics, the knowledge of the microstructure is of importance, since it
essentially determines the stability of a workpiece to be produced. To
enlighten this topic, we investigate these suspensions under shear by means of
simulations. We observe cluster formation on two different length scales: the
distance of nearest neighbors and on the length scale of the system size. We
find that the clustering behavior does not depend on the length scale of
observation. If inter-particle interactions are not attractive the particles
form layers in the shear flow. The results are summarized in a stability
diagram.Comment: 15 pages, 10 figures, revised versio
Spin models for orientational ordering of colloidal molecular crystals
Two-dimensional colloidal suspensions exposed to periodic external fields
exhibit a variety of molecular crystalline phases. There two or more colloids
assemble at lattice sites of potential minima to build new structural entities,
referred to as molecules. Using the strength of the potential and the filling
fraction as control parameter, phase transition to unconventional
orientationally ordered states can be induced. We introduce an approach that
focuses at the discrete set of orientational states relevant for the phase
ordering. The orientationally ordered states are mapped to classical spin
systems. We construct effective hamiltonians for dimeric and trimeric molecules
on triangular lattices suitable for a statistical mechanics discussion. A
mean-field analysis produces a rich phase behavior which is substantiated by
Monte Carlo simulations.Comment: 19 pages, 21 figures; misplacement of Fig.3 fixe
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
Charge-Reversal Instability in Mixed Bilayer Vesicles
Bilayer vesicles form readily from mixtures of charged and neutral
surfactants. When such a mixed vesicle binds an oppositely-charged object, its
membrane partially demixes: the adhesion zone recruits more charged surfactants
from the rest of the membrane. Given an unlimited supply of adhering objects
one might expect the vesicle to remain attractive until it was completely
covered. Contrary to this expectation, we show that a vesicle can instead
exhibit {\it adhesion saturation,} partitioning spontaneously into an
attractive zone with definite area fraction, and a repulsive zone. The latter
zone rejects additional incoming objects because counterions on the interior of
the vesicle migrate there, effectively reversing the membrane's charge. The
effect is strongest at high surface charge densities, low ionic strength, and
with thin, impermeable membranes. Adhesion saturation in such a situation has
recently been observed experimentally [H. Aranda-Espinoza {\it et al.}, {\sl
Science} {\bf285} 394--397 (1999)]
Direct measurement of three-body interactions
Three-body interactions have been measured among three charged colloidal
particles in deionized solvent. Two of the particles have been confined to an
optical line-trap while the third one was approached by means of a focused
laser beam. The experimentally determined three-body interactions are
attractive and roughly of the same magnitude and range as the
pair-interactions. In addition, numerical calculations have been performed,
which show good agreement with the experimental results
Attraction between DNA molecules mediated by multivalent ions
The effective force between two parallel DNA molecules is calculated as a
function of their mutual separation for different valencies of counter- and
salt ions and different salt concentrations. Computer simulations of the
primitive model are used and the shape of the DNA molecules is accurately
modelled using different geometrical shapes. We find that multivalent ions
induce a significant attraction between the DNA molecules whose strength can be
tuned by the averaged valency of the ions. The physical origin of the
attraction is traced back either to electrostatics or to entropic
contributions. For multivalent counter- and monovalent salt ions, we find a
salt-induced stabilization effect: the force is first attractive but gets
repulsive for increasing salt concentration. Furthermore, we show that the
multivalent-ion-induced attraction does not necessarily correlate with DNA
overcharging.Comment: 51 pages and 13 figure
Directing Brownian motion by oscillating barriers
Bleil S, Reimann P, Bechinger C. Directing Brownian motion by oscillating barriers. PHYSICAL REVIEW E. 2007;75(3): 031117.We consider the Brownian motion of a colloidal particle in a symmetric, periodic potential, whose potential barriers are subjected to temporal oscillations. Experimentally, the potential is generated by two arrays of trapped, negatively charged particles whose positions are periodically modulated with light forces. This results in a structured channel geometry of locally variable width. If all potential barriers are oscillating in synchrony, a resonancelike peak of the effective diffusion coefficient upon variation of the oscillation period is observed. For asynchronously oscillating barriers, the particle can be steered with great reliability into one or the other direction by properly choosing the oscillation periods of the different barriers along the channel