47 research outputs found
The short-time self-diffusion coefficient of a sphere in a suspension of rigid rods
The short--time self diffusion coefficient of a sphere in a suspension of
rigid rods is calculated in first order in the rod volume fraction. For low rod
concentrations the correction to the Einstein diffusion constant of the sphere
is a linear function of the rod volume fraction with the slope proportional to
the equilibrium averaged mobility diminution trace of the sphere interacting
with a single freely translating and rotating rod. The two--body hydrodynamic
interactions are calculated using the so--called bead model in which the rod is
replaced by a stiff linear chain of touching spheres. The interactions between
spheres are calculated numerically using the multipole method. Also an
analytical expression for the diffusion coefficient as a function of the rod
aspect ratio is derived in the limit of very long rods. We show that in this
limit the correction to the Einstein diffusion constant does not depend on the
size of the tracer sphere. The higher order corrections depending on the
applied model are computed numerically. An approximate expression is provided,
valid for a wide range of aspect ratios.Comment: 11 pages, 6 figure
Rotational and translational self-diffusion in concentrated suspensions of permeable particles
In our recent work on concentrated suspensions of uniformly porous colloidal
spheres with excluded volume interactions, a variety of short-time dynamic
properties were calculated, except for the rotational self-diffusion
coefficient. This missing quantity is included in the present paper. Using a
precise hydrodynamic force multipole simulation method, the rotational
self-diffusion coefficient is evaluated for concentrated suspensions of
permeable particles. Results are presented for particle volume fractions up to
45%, and for a wide range of permeability values. From the simulation results
and earlier results for the first-order virial coefficient, we find that the
rotational self-diffusion coefficient of permeable spheres can be scaled to the
corresponding coefficient of impermeable particles of the same size. We also
show that a similar scaling applies to the translational self-diffusion
coefficient considered earlier. From the scaling relations, accurate analytic
approximations for the rotational and translational self-diffusion coefficients
in concentrated systems are obtained, useful to the experimental analysis of
permeable-particle diffusion. The simulation results for rotational diffusion
of permeable particles are used to show that a generalized
Stokes-Einstein-Debye relation between rotational self-diffusion coefficient
and high-frequency viscosity is not satisfied.Comment: 4 figure
eficiência de piraclostrobina como indutor de resistência em cana-de-açúcar a meloidoginose
O presente trabalho teve como objetivo estudar a eficiência da indução de resistência como nova alternativa no manejo integrado em cana-de-açúcar no estado de Pernambuco
A General Approach for Predicting the Filtration of Soft and Permeable Colloids: The Milk Example
Membrane filtration operations (ultra-, microfiltration) are now extensively used for concentrating or separating an ever-growing variety of colloidal dispersions. However, the phenomena that determine the efficiency of these operations are not yet fully understood. This is especially the case when dealing with colloids that are soft, deformable, and permeable. In this paper, we propose a methodology for building a model that is able to predict the performance (flux, concentration profiles) of the filtration of such objects in relation with the operating conditions. This is done by focusing on the case of milk filtration, all experiments being performed with dispersions of milk casein micelles, which are sort of ″natural″ colloidal microgels. Using this example, we develop the general idea that a filtration model can always be built for a given colloidal dispersion as long as this dispersion has been characterized in terms of osmotic pressure Πand hydraulic permeability k. For soft and permeable colloids, the major issue is that the permeability k cannot be assessed in a trivial way like in the case for hard-sphere colloids. To get around this difficulty, we follow two distinct approaches to actually measure k: a direct approach, involving osmotic stress experiments, and a reverse-calculation approach, that consists of estimating k through well-controlled filtration experiments. The resulting filtration model is then validated against experimental measurements obtained from combined milk filtration/SAXS experiments. We also give precise examples of how the model can be used, as well as a brief discussion on the possible universality of the approach presented here
Bagaza virus in wild birds, Portugal, 2021
Bagaza virus emerged in Spain in 2010 and was not reported in other countries in Europe until 2021, when the virus was detected by molecular methods in a corn bunting and several red-legged partridges in Portugal. Sequencing revealed high similarity between the 2021 strains from Portugal and the 2010 strains from Spain.Peer reviewe
Soft Dynamics simulation: 2. Elastic spheres undergoing a T1 process in a viscous fluid
Robust empirical constitutive laws for granular materials in air or in a
viscous fluid have been expressed in terms of timescales based on the dynamics
of a single particle. However, some behaviours such as viscosity bifurcation or
shear localization, observed also in foams, emulsions, and block copolymer
cubic phases, seem to involve other micro-timescales which may be related to
the dynamics of local particle reorganizations. In the present work, we
consider a T1 process as an example of a rearrangement. Using the Soft dynamics
simulation method introduced in the first paper of this series, we describe
theoretically and numerically the motion of four elastic spheres in a viscous
fluid. Hydrodynamic interactions are described at the level of lubrication
(Poiseuille squeezing and Couette shear flow) and the elastic deflection of the
particle surface is modeled as Hertzian. The duration of the simulated T1
process can vary substantially as a consequence of minute changes in the
initial separations, consistently with predictions. For the first time, a
collective behaviour is thus found to depend on another parameter than the
typical volume fraction in particles.Comment: 11 pages - 5 figure
Epidemiological and clinical characteristics of the COVID-19 epidemic in Brazil
The first case of COVID-19 was detected in Brazil on 25 February 2020. We report and contextualize epidemiological, demographic and clinical findings for COVID-19 cases during the first 3 months of the epidemic. By 31 May 2020, 514,200 COVID-19 cases, including 29,314 deaths, had been reported in 75.3% (4,196 of 5,570) of municipalities across all five administrative regions of Brazil. The R0 value for Brazil was estimated at 3.1 (95% Bayesian credible interval = 2.4–5.5), with a higher median but overlapping credible intervals compared with some other seriously affected countries. A positive association between higher per-capita income and COVID-19 diagnosis was identified. Furthermore, the severe acute respiratory infection cases with unknown aetiology were associated with lower per-capita income. Co-circulation of six respiratory viruses was detected but at very low levels. These findings provide a comprehensive description of the ongoing COVID-19 epidemic in Brazil and may help to guide subsequent measures to control virus transmission