683 research outputs found
Non-equilibrium sedimentation of colloids on the particle scale
We investigate sedimentation of model hard sphere-like colloidal dispersions
confined in horizontal capillaries using laser scanning confocal microscopy,
dynamical density functional theory, and Brownian dynamics computer
simulations. For homogenized initial states we obtain quantitative agreement of
the results from the respective approaches for the time evolution of the
one-body density distribution and the osmotic pressure on the walls. We
demonstrate that single particle information can be obtained experimentally in
systems that were initialized further out-of-equilibrium such that complex
lateral patterns form.Comment: to be published in Phys. Rev. Let
On-chip electrically controlled routing of photons from a single quantum dot
Electrical control of on-chip routing of photons emitted by a single InAs/GaAs self-assembled quantum dot (SAQD) is demonstrated in a photonic crystal cavity-waveguide system. The SAQD is located inside an H1 cavity, which is coupled to two photonic crystal waveguides. The SAQD emission wavelength is electrically tunable by the quantum-confined Stark effect. When the SAQD emission is brought into resonance with one of two H1 cavity modes, it is preferentially routed to the waveguide to which that mode is selectively coupled. This proof of concept provides the basis for scalable, low-power, high-speed operation of single-photon routers for use in integrated quantum photonic circuits
Colloidal brazil nut effect in sediments of binary charged suspensions
Equilibrium sedimentation density profiles of charged binary colloidal
suspensions are calculated by computer simulations and density functional
theory. For deionized samples, we predict a colloidal ``brazil nut'' effect:
heavy colloidal particles sediment on top of the lighter ones provided that
their mass per charge is smaller than that of the lighter ones. This effect is
verifiable in settling experiments.Comment: 4 pages, 4 figure
The sediment of mixtures of charged colloids: segregation and inhomogeneous electric fields
We theoretically study sedimentation-diffusion equilibrium of dilute binary,
ternary, and polydisperse mixtures of colloidal particles with different
buoyant masses and/or charges. We focus on the low-salt regime, where the
entropy of the screening ions drives spontaneous charge separation and the
formation of an inhomogeneous macroscopic electric field. The resulting
electric force lifts the colloids against gravity, yielding highly
nonbarometric and even nonmonotonic colloidal density profiles. The most
profound effect is the phenomenon of segregation into layers of colloids with
equal mass-per-charge, including the possibility that heavy colloidal species
float onto lighter ones
Dynamical density functional theory for dense atomic liquids
Starting from Newton's equations of motion, we derive a dynamical density
functional theory (DDFT) applicable to atomic liquids. The theory has the
feature that it requires as input the Helmholtz free energy functional from
equilibrium density functional theory. This means that, given a reliable
equilibrium free energy functional, the correct equilibrium fluid density
profile is guaranteed. We show that when the isothermal compressibility is
small, the DDFT generates the correct value for the speed of sound in a dense
liquid. We also interpret the theory as a dynamical equation for a coarse
grained fluid density and show that the theory can be used (making further
approximations) to derive the standard mode coupling theory that is used to
describe the glass transition. The present theory should provide a useful
starting point for describing the dynamics of inhomogeneous atomic fluids.Comment: 14 pages, accepted for publication in J. Phys.: Condens. Matte
NOX AND VOC MEASUREMENTS AND HEALTH RISK ASSESSMENT IN AN INFORMAL SETTLEMENT IN DURBAN
A previous study by Muller et al. (2003) investigated NOx, benzene, toluene, ethylbenzene and xylene levels in households in the densely settled informal settlement of Cato Crest located within the Durban Metropolitan area. AHealth Risk Assessment based on the United States EPA approach showed that the residents of Cato Crest experienced significant health risks as a result of exposure to these pollutants largely as a result of kerosene usage in their homes. Specifically, the study which was conducted in September 2000 in 14 households, showed that exposure to NOx over a 24-hour period indicated a potential health risk in all the households, that benzene poses a health risk in 50% of the households, whereas there is no health risk associated with exposure to toluene. A follow-up study was conducted in July 2006 targeting similar households and pollutants to investigate whether comparable results were found in a typical winter period, when air quality is generally expected to deteriorate
Extended sedimentation profiles in charged colloids: the gravitational length, entropy, and electrostatics
We have measured equilibrium sedimentation profiles in a colloidal model
system with confocal microscopy. By tuning the interactions, we have determined
the gravitational length in the limit of hard-sphere-like interactions, and
using the same particles, tested a recent theory [R.van Roij, J. Phys. Cond.
Mat. 15, S3569, (2003)], which predicts a significantly extended sedimentation
profile in the case of charged colloids with long-ranged repulsions, due to a
spontaneously formed macroscopic electric field. For the hard-sphere-like
system we find that the gravitational length matches that expected. By tuning
the buoyancy of the colloidal particles we have shown that a mean field
hydrostatic equilibrium description even appears to hold in the case that the
colloid volume fraction changes significantly on the length scale of the
particle size. The extended sedimentation profiles of the colloids with
long-ranged repulsions are well-described by theory. Surprisingly, the theory
even seems to hold at concentrations where interactions between the colloids,
which are not modeled explicitly, play a considerable role
Geometric frustration in small colloidal clusters
We study the structure of clusters in a model colloidal system with competing
interactions using Brownian dynamics simulations. A short-ranged attraction
drives clustering, while a weak, long-ranged repulsion is used to model
electrostatic charging in experimental systems. The former is treated with a
short-ranged Morse attractive interaction, the latter with a repulsive Yukawa
interaction. We consider the yield of clusters of specific structure as a
function of the strength of the interactions, for clusters with m=3,4,5,6,7,10
and 13 colloids. At sufficient strengths of the attractive interaction (around
10 kT), the average bond lifetime approaches the simulation timescale and the
system becomes nonergodic. For small clusters m<=5 where geometric frustration
is not relevant, despite nonergodicity, for sufficient strengths of the
attractive interaction the yield of clusters which maximise the number of bonds
approaches 100%. However for and higher, in the nonergodic regime we find
a lower yield of these structures where we argue geometric frustration plays a
significant role. is a special case, where two structures, of octahedral
and C2v symmetry compete, with the latter being favoured by entropic
contributions in the ergodic regime and by kinetic trapping in the nonergodic
regime. We believe that our results should be valid as far as the one-component
description of the interaction potential is valid. A system with competing
electrostatic repulsions and van der Waals attractions may be such an example.
However, in some cases, the one-component description of the interaction
potential may not be appropriate.Comment: 21 pages, accepted for publication by J. Phys. Condens. Matte
Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?
We study sedimentation equilibrium of low-salt suspensions of binary mixtures
of charged colloids, both by Monte Carlo simulations of an effective
colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture.
We show that the theoretically predicted lifting and layering effect, which
involves the entropy of the screening ions and a spontaneous macroscopic
electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480
(2005)], can also be understood on the basis of an effective colloid-only
system with pairwise screened-Coulomb interactions. We consider, by theory and
by simulation, both repelling like-charged colloids and attracting oppositely
charged colloids, and we find a re-entrant lifting and layering phenomenon when
the charge ratio of the colloids varies from large positive through zero to
large negative values
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