1,864 research outputs found
Stochastic Generation of Particle Structures with Controlled Degree of Heterogeneity
The recently developed void expansion method (VEM) allows for an efficient
generation of porous packings of spherical particles over a wide range of
volume fractions. The method is based on a random placement of the structural
particles under addition of much smaller "void-particles" whose radii are
repeatedly increased during the void expansion. Thereby, they rearrange the
structural particles until formation of a dense particle packing and introduce
local heterogeneities in the structure. In this paper, microstructures with
volume fractions between 0.4 and 0.6 produced by VEM are analyzed with respect
to their degree of heterogeneity (DOH). In particular, the influence of the
void- to structural particle number ratio, which constitutes a principal
VEM-parameter, on the DOH is studied. The DOH is quantified using the pore size
distribution, the Voronoi volume distribution and the density-fluctuation
method in conjunction with fit functions or integral measures. This analysis
has revealed that for volume fractions between 0.4 and 0.55 the void-particle
number allows for a quasi-continuous adjustment of the DOH. Additionally, the
DOH-range of VEM-generated microstructures with a volume fraction of 0.4 is
compared to the range covered by microstructures generated using previous
Brownian dynamics simulations, which represent the structure of coagulated
colloidal suspensions. Both sets of microstructures cover similarly broad and
overlapping DOH-ranges, which allows concluding that VEM is an efficient method
to stochastically reproduce colloidal microstructures with varying DOH.Comment: 10 pages, 7 figure
PSD-95 Is Associated with the Postsynaptic Density and Not with the Presynaptic Membrane at Forebrain Synapses
PSD-95, a prominent protein component of the postsynaptic density (PSD) fraction from rat forebrain, has been localized by light microscopy to dendrites of hippocampal neurons (Cho et al., 1992) and to the presynaptic plexus of cerebellar basket cells (Kistner et al., 1993). Here we extend these studies to show that an affinity-purified antibody to PSD-95 labels the dendrites of most neurons in the forebrain and of a subset of neurons in the cerebellum. To confirm that PSD-95 is associated with the PSD at forebrain synapses and to clarify whether it is also associated with the presynaptic membrane, we employed immunogold electron microscopy of forebrain synaptosomes. Gold-labeled antibodies to PSD-95 labeled postsynaptic densities in both intact and lysed forebrain synaptosomes but did not label presynaptic terminals or the presynaptic membrane. The asymmetric distribution of PSD-95 at synapses contrasts with that of its homologs, disks-large and ZO-1, which are arranged symmetrically at septate and tight junctions, respectively
Microstructures and Mechanical Properties of Dense Particle Gels: Microstructural Characterization
The macroscopic mechanical properties of densely packed coagulated colloidal
particle gels strongly depend on the local arrangement of the powder particles
on length scales of a few particle diameters. Heterogeneous microstructures
exhibit up to one order of magnitude higher elastic properties and yield
strengths than their homogeneous counterparts. The microstructures of these
gels are analyzed by the straight path method quantifying quasi-linear particle
arrangements of particles. They show similar characteristics than force chains
bearing the mechanical load in granular material. Applying this concept to gels
revealed that heterogeneous colloidal microstructures show a significantly
higher straight paths density and exhibit longer straight paths than their
homogeneous counterparts.Comment: 7 pages, 9 figure
Blunting the Spike: the CV Minimum Period
The standard picture of CV secular evolution predicts a spike in the CV
distribution near the observed short-period cutoff P_0 ~ 78 min, which is not
observed. We show that an intrinsic spread in minimum (`bounce') periods P_b
resulting from a genuine difference in some parameter controlling the evolution
can remove the spike without smearing the sharpness of the cutoff. The most
probable second parameter is different admixtures of magnetic stellar wind
braking (at up to 5 times the GR rate) in a small tail of systems, perhaps
implying that the donor magnetic field strength at formation is a second
parameter specifying CV evolution. We suggest that magnetic braking resumes
below the gap with a wide range, being well below the GR rate in most CVs, but
significantly above it in a small tail.Comment: 5 pages, 4 figures; accepted for publication in MNRA
Spectral Theory of Time Dispersive and Dissipative Systems
We study linear time dispersive and dissipative systems. Very often such
systems are not conservative and the standard spectral theory can not be
applied. We develop a mathematically consistent framework allowing (i) to
constructively determine if a given time dispersive system can be extended to a
conservative one; (ii) to construct that very conservative system -- which we
show is essentially unique. We illustrate the method by applying it to the
spectral analysis of time dispersive dielectrics and the damped oscillator with
retarded friction. In particular, we obtain a conservative extension of the
Maxwell equations which is equivalent to the original Maxwell equations for a
dispersive and lossy dielectric medium.Comment: LaTeX, 57 Pages, incorporated revisions corresponding with published
versio
Challenging otherness : a reassessment of early Greek attitudes toward the divine
Includes bibliographical references
Keck Spectroscopy of Faint 3<z<8 Lyman Break Galaxies:- Evidence for a Declining Fraction of Emission Line Sources In the Redshift Range 6<z<8
Using deep Keck spectroscopy of Lyman break galaxies selected from infrared
imaging data taken with WFC3/IR onboard the Hubble Space Telescope, we present
new evidence for a reversal in the redshift-dependent fraction of star forming
galaxies with detectable Lyman alpha emission in the redshift range 6.3 < z <
8.8. Our earlier surveys with the DEIMOS spectrograph demonstrated a
significant increase with redshift in the fraction of line emitting galaxies
over the interval 4 < z < 6, particularly for intrinsically faint systems which
dominate the luminosity density. Using the longer wavelength sensitivities of
LRIS and NIRSPEC, we have targeted 19 Lyman break galaxies selected using
recent WFC3/IR data whose photometric redshifts are in the range 6.3 < z < 8.8
and which span a wide range of intrinsic luminosities. Our spectroscopic
exposures typically reach a 5-sigma sensitivity of < 50 A for the rest-frame
equivalent width (EW) of Lyman alpha emission. Despite the high fraction of
emitters seen only a few hundred million years later, we find only 2 convincing
and 1 possible line emitter in our more distant sample. Combining with
published data on a further 7 sources obtained using FORS2 on the ESO VLT, and
assuming continuity in the trends found at lower redshift, we discuss the
significance of this apparent reversal in the redshift-dependent Lyman alpha
fraction in the context of our range in continuum luminosity. Assuming all the
targeted sources are at their photometric redshift and our assumptions about
the Lyman alpha EW distribution are correct, we would expect to find so few
emitters in less than 1% of the realizations drawn from our lower redshift
samples. Our new results provide further support for the suggestion that, at
the redshifts now being probed spectroscopically, we are entering the era where
the intergalactic medium is partially neutral.Comment: 8 pages, 5 figures, Accepted to ApJ 10/1/1
The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings
The macroscopic mechanical properties of colloidal particle gels strongly
depend on the local arrangement of the powder particles. Experiments have shown
that more heterogeneous microstructures exhibit up to one order of magnitude
higher elastic properties than their more homogeneous counterparts at equal
volume fraction. In this paper, packings of spherical particles are used as
model structures to computationally investigate the elastic properties of
coagulated particle gels as a function of their degree of heterogeneity. The
discrete element model comprises a linear elastic contact law, particle bonding
and damping. The simulation parameters were calibrated using a homogeneous and
a heterogeneous microstructure originating from earlier Brownian dynamics
simulations. A systematic study of the elastic properties as a function of the
degree of heterogeneity was performed using two sets of microstructures
obtained from Brownian dynamics simulation and from the void expansion method.
Both sets cover a broad and to a large extent overlapping range of degrees of
heterogeneity. The simulations have shown that the elastic properties as a
function of the degree of heterogeneity are independent of the structure
generation algorithm and that the relation between the shear modulus and the
degree of heterogeneity can be well described by a power law. This suggests the
presence of a critical degree of heterogeneity and, therefore, a phase
transition between a phase with finite and one with zero elastic properties.Comment: 8 pages, 6 figures; Granular Matter (published online: 11. February
2012
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