382 research outputs found
Density fluctuations and the structure of a nonuniform hard sphere fluid
We derive an exact equation for density changes induced by a general external
field that corrects the hydrostatic approximation where the local value of the
field is adsorbed into a modified chemical potential. Using linear response
theory to relate density changes self-consistently in different regions of
space, we arrive at an integral equation for a hard sphere fluid that is exact
in the limit of a slowly varying field or at low density and reduces to the
accurate Percus-Yevick equation for a hard core field. This and related
equations give accurate results for a wide variety of fields
Gel transitions in colloidal suspensions
The idealized mode coupling theory (MCT) is applied to colloidal systems
interacting via short-range attractive interactions of Yukawa form. At low
temperatures MCT predicts a slowing down of the local dynamics and ergodicity
breaking transitions. The nonergodicity transitions share many features with
the colloidal gel transition, and are proposed to be the source of gelation in
colloidal systems. Previous calculations of the phase diagram are complemented
with additional data for shorter ranges of the attractive interaction, showing
that the path of the nonergodicity transition line is then unimpeded by the
gas-liquid critical curve at low temperatures. Particular attention is given to
the critical nonergodicity parameters, motivated by recent experimental
measurements. An asymptotic model is developed, valid for dilute systems of
spheres interacting via strong short-range attractions, and is shown to capture
all aspects of the low temperature MCT nonergodicity transitions.Comment: 12 pages, LaTeX, 5 eps figures, uses ioplppt.sty, to appear in J.
Phys.: Condens. Matte
Theory and simulation of short-range models of globular protein solutions
We report theoretical and simulation studies of phase coexistence in model
globular protein solutions, based on short-range, central, pair potential
representations of the interaction among macro-particles. After reviewing our
previous investigations of hard-core Yukawa and generalised Lennard-Jones
potentials, we report more recent results obtained within a DLVO-like
description of lysozyme solutions in water and added salt. We show that a
one-parameter fit of this model based on Static Light Scattering and
Self-Interaction Chromatography data in the dilute protein regime, yields
demixing and crystallization curves in good agreement with experimental
protein-rich/protein-poor and solubility envelopes. The dependence of cloud and
solubility points temperature of the model on the ionic strength is also
investigated. Our findings highlight the minimal assumptions on the properties
of the microscopic interaction sufficient for a satisfactory reproduction of
the phase diagram topology of globular protein solutions.Comment: 17 pages, 8 figures, Proc. of Conference "Structural Arrest
Transitions in Colloidal Systems with Short-Range Attractions", Messina
(ITALY) 17-20 December 200
The liquid-vapor interface of an ionic fluid
We investigate the liquid-vapor interface of the restricted primitive model
(RPM) for an ionic fluid using a density-functional approximation based on
correlation functions of the homogeneous fluid as obtained from the
mean-spherical approximation (MSA). In the limit of a homogeneous fluid our
approach yields the well-known MSA (energy) equation of state. The ionic
interfacial density profiles, which for the RPM are identical for both species,
have a shape similar to those of simple atomic fluids in that the decay towards
the bulk values is more rapid on the vapor side than on the liquid side. This
is the opposite asymmetry of the decay to that found in earlier calculations
for the RPM based on a square-gradient theory. The width of the interface is,
for a wide range of temperatures, approximately four times the second moment
correlation length of the liquid phase. We discuss the magnitude and
temperature dependence of the surface tension, and argue that for temperatures
near the triple point the ratio of the dimensionless surface tension and
critical temperature is much smaller for the RPM than for simple atomic fluids.Comment: 6 postscript figures, submitted to Phys. Rev.
A mind you can count on: validating breath counting as a behavioral measure of mindfulness.
Mindfulness practice of present moment awareness promises many benefits, but has eluded rigorous behavioral measurement. To date, research has relied on self-reported mindfulness or heterogeneous mindfulness trainings to infer skillful mindfulness practice and its effects. In four independent studies with over 400 total participants, we present the first construct validation of a behavioral measure of mindfulness, breath counting. We found it was reliable, correlated with self-reported mindfulness, differentiated long-term meditators from age-matched controls, and was distinct from sustained attention and working memory measures. In addition, we employed breath counting to test the nomological network of mindfulness. As theorized, we found skill in breath counting associated with more meta-awareness, less mind wandering, better mood, and greater non-attachment (i.e., less attentional capture by distractors formerly paired with reward). We also found in a randomized online training study that 4 weeks of breath counting training improved mindfulness and decreased mind wandering relative to working memory training and no training controls. Together, these findings provide the first evidence for breath counting as a behavioral measure of mindfulness
Nonergodicity transitions in colloidal suspensions with attractive interactions
The colloidal gel and glass transitions are investigated using the idealized
mode coupling theory (MCT) for model systems characterized by short-range
attractive interactions. Results are presented for the adhesive hard sphere and
hard core attractive Yukawa systems. According to MCT, the former system shows
a critical glass transition concentration that increases significantly with
introduction of a weak attraction. For the latter attractive Yukawa system, MCT
predicts low temperature nonergodic states that extend to the critical and
subcritical region. Several features of the MCT nonergodicity transition in
this system agree qualitatively with experimental observations on the colloidal
gel transition, suggesting that the gel transition is caused by a low
temperature extension of the glass transition. The range of the attraction is
shown to govern the way the glass transition line traverses the phase diagram
relative to the critical point, analogous to findings for the fluid-solid
freezing transition.Comment: 11 pages, 7 figures; to be published in Phys. Rev. E (1 May 1999
Rapid tests and urine sampling techniques for the diagnosis of urinary tract infection (UTI) in children under five years: a systematic review
Background: Urinary tract infection (UTI) is one of the most common sources of infection in children under five. Prompt diagnosis and treatment is important to reduce the risk of renal scarring. Rapid, cost-effective, methods of UTI diagnosis are required as an alternative to culture. Methods: We conducted a systematic review to determine the diagnostic accuracy of rapid tests for detecting UTI in children under five years of age. Results: The evidence supports the use of dipstick positive for both leukocyte esterase and nitrite (pooled LR+ = 28.2, 95% CI: 17.3, 46.0) or microscopy positive for both pyuria and bacteriuria (pooled LR+ = 37.0, 95% CI: 11.0, 125.9) to rule in UTI. Similarly dipstick negative for both LE and nitrite (Pooled LR- = 0.20, 95% CI: 0.16, 0.26) or microscopy negative for both pyuria and bacteriuria (Pooled LR- = 0.11, 95% CI: 0.05, 0.23) can be used to rule out UTI. A test for glucose showed promise in potty-trained children. However, all studies were over 30 years old. Further evaluation of this test may be useful. Conclusion: Dipstick negative for both LE and nitrite or microscopic analysis negative for both pyuria and bacteriuria of a clean voided urine, bag, or nappy/pad specimen may reasonably be used to rule out UTI. These patients can then reasonably be excluded from further investigation, without the need for confirmatory culture. Similarly, combinations of positive tests could be used to rule in UTI, and trigger further investigation
Phase equilibria and glass transition in colloidal systems with short-ranged attractive interactions. Application to protein crystallization
We have studied a model of a complex fluid consisting of particles
interacting through a hard core and a short range attractive potential of both
Yukawa and square-well form. Using a hybrid method, including a self-consistent
and quite accurate approximation for the liquid integral equation in the case
of the Yukawa fluid, perturbation theory to evaluate the crystal free energies,
and mode-coupling theory of the glass transition, we determine both the
equilibrium phase diagram of the system and the lines of equilibrium between
the supercooled fluid and the glass phases. For these potentials, we study the
phase diagrams for different values of the potential range, the ratio of the
range of the interaction to the diameter of the repulsive core being the main
control parameter. Our arguments are relevant to a variety of systems, from
dense colloidal systems with depletion forces, through particle gels,
nano-particle aggregation, and globular protein crystallization.Comment: 20 pages, 10 figure
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