2,732 research outputs found
Remote sensing applications in forestry. The development of an earth resources information system using aerial photographs and digital computers photographs and digital computers
Remote aerial sensing and automatic mapping for forest resources information syste
Influence of shear stress applied during flow stoppage and rest period on the mechanical properties of thixotropic suspensions
We study the solid mechanical properties of several thixotropic suspensions
as a function of the shear stress history applied during their flow stoppage
and their aging in their solid state. We show that their elastic modulus and
yield stress depend strongly on the shear stress applied during their
solid-liquid transition (i.e., during flow stoppage) while applying the same
stress only before or only after this transition may induce only second-order
effects: there is negligible dependence of the mechanical properties on the
preshear history and on the shear stress applied at rest. We also found that
the suspensions age with a structuration rate that hardly depends on the stress
history. We propose a physical sketch based on the freezing of a microstructure
whose anisotropy depends on the stress applied during the liquid-solid
transition to explain why the mechanical properties depend strongly on this
stress. This sketch points out the role of the internal forces in the colloidal
suspensions' behavior. We finally discuss briefly the macroscopic consequences
of this phenomenon and show the importance of using a controlled-stress
rheometer
Reverse-selective diffusion in nanocomposite membranes
The permeability of certain polymer membranes with impenetrable
nanoinclusions increases with the particle volume fraction (Merkel et al.,
Science, 296, 2002). This intriguing observation contradicts even qualitative
expectations based on Maxwell's classical theory of conduction/diffusion in
composites with homogeneous phases. This letter presents a simple theoretical
interpretation based on classical models of diffusion and polymer physics. An
essential feature of the theory is a polymer-segment depletion layer at the
inclusion-polymer interface. The accompanying increase in free volume leads to
a significant increase in the local penetrant diffusivity, which, in turn,
increases the bulk permeability while exhibiting reverse selectivity. This
model captures the observed dependence of the bulk permeability on the
inclusion size and volume fraction, providing a straightforward connection
between membrane microstructure and performance
Micro-evaporators for kinetic exploration of phase diagrams
We use pervaporation-based microfluidic devices to concentrate species in
aqueous solutions with spatial and temporal control of the process. Using
experiments and modelling, we quantitatively describe the advection-diffusion
behavior of the concentration field of various solutions (electrolytes,
colloids, etc) and demonstrate the potential of these devices as universal
tools for the kinetic exploration of the phases and textures that form upon
concentration
Integration through transients for Brownian particles under steady shear
Starting from the microscopic Smoluchowski equation for interacting Brownian
particles under stationary shearing, exact expressions for shear-dependent
steady-state averages, correlation and structure functions, and
susceptibilities are obtained, which take the form of generalized Green-Kubo
relations. They require integration of transient dynamics. Equations of motion
with memory effects for transient density fluctuation functions are derived
from the same microscopic starting point. We argue that the derived formal
expressions provide useful starting points for approximations in order to
describe the stationary non-equilibrium state of steadily sheared dense
colloidal dispersions.Comment: 17 pages, Submitted to J. Phys.: Condens. Matter; revised version
with minor correction
Electric-field-induced displacement of a charged spherical colloid embedded in an elastic Brinkman medium
When an electric field is applied to an electrolyte-saturated polymer gel
embedded with charged colloidal particles, the force that must be exerted by
the hydrogel on each particle reflects a delicate balance of electrical,
hydrodynamic and elastic stresses. This paper examines the displacement of a
single charged spherical inclusion embedded in an uncharged hydrogel. We
present numerically exact solutions of coupled electrokinetic transport and
elastic-deformation equations, where the gel is treated as an incompressible,
elastic Brinkman medium. This model problem demonstrates how the displacement
depends on the particle size and charge, the electrolyte ionic strength, and
Young's modulus of the polymer skeleton. The numerics are verified, in part,
with an analytical (boundary-layer) theory valid when the Debye length is much
smaller than the particle radius. Further, we identify a close connection
between the displacement when a colloid is immobilized in a gel and its
velocity when dispersed in a Newtonian electrolyte. Finally, we describe an
experiment where nanometer-scale displacements might be accurately measured
using back-focal-plane interferometry. The purpose of such an experiment is to
probe physicochemical and rheological characteristics of hydrogel composites,
possibly during gelation
Universal reduction of pressure between charged surfaces by long-wavelength surface charge modulation
We predict theoretically that long-wavelength surface charge modulations
universally reduce the pressure between the charged surfaces with counterions
compared with the case of uniformly charged surfaces with the same average
surface charge density. The physical origin of this effect is the fact that
surface charge modulations always lead to enhanced counterion localization near
the surfaces, and hence, fewer charges at the midplane. We confirm the last
prediction with Monte Carlo simulations.Comment: 8 pages 1 figure, Europhys. Lett., in pres
Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high resolution optical spectra obtained with the HIRES
spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs
(LMTTs) in Taurus-Auriga. Of particular interest is the previously classified
"continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass
just below the stellar/substellar boundary. None of the LMTTs in Taurus are
rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of
the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent
H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We
demonstrate empirically that the full-width at 10% of the H-alpha emission
profile peak is a more practical and possibly more accurate indicator of
accretion than either the equivalent width of H-alpha or optical veiling:
10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent
of stellar spectral type. Although LMTTs can have accretion rates comparable to
that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass
accretion rate appears to decrease with decreasing mass. The diminished
frequency of accretion disks for LMTTs, in conjunction with their lower, on
average, mass accretion rates, implies that they are formed with less massive
disks than higher-mass T Tauri stars. The radial velocities, circumstellar
properties and known binaries do not support the suggestion that many of the
lowest mass members of Taurus have been ejected from higher stellar density
regions within the cloud. Instead, LMTTs appear to have formed and are evolving
in the same way as higher-mass T Tauri stars, but with smaller disks and
shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap
Simulation of Claylike Colloids
We investigate properties of dense suspensions and sediments of small
spherical silt particles by means of a combined Molecular Dynamics (MD) and
Stochastic Rotation Dynamics (SRD) simulation. We include van der Waals and
effective electrostatic interactions between the colloidal particles, as well
as Brownian motion and hydrodynamic interactions which are calculated in the
SRD-part. We present the simulation technique and first results. We have
measured velocity distributions, diffusion coefficients, sedimentation
velocity, spatial correlation functions and we have explored the phase diagram
depending on the parameters of the potentials and on the volume fraction.Comment: 20 pages, 14 figure
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