1,881 research outputs found
Pattern Formation by Boundary Forcing in Convectively Unstable, Oscillatory Media With and Without Differential Transport
Motivated by recent experiments and models of biological segmentation, we
analyze the exicitation of pattern-forming instabilities of convectively
unstable reaction-diffusion-advection (RDA) systems, occuring by means of
constant or periodic forcing at the upstream boundary. Such boundary-controlled
pattern selection is a generalization of the flow-distributed oscillation (FDO)
mechanism that can include Turing or differential flow instability (DIFI)
modes. Our goal is to clarify the relationships among these mechanisms in the
general case where there is differential flow as well as differential
diffusion. We do so by analyzing the dispersion relation for linear
perturbations and showing how its solutions are affected by differential
transport. We find a close relationship between DIFI and FDO, while the Turing
mechanism gives rise to a distinct set of unstable modes. Finally, we
illustrate the relevance of the dispersion relations using nonlinear
simulations and we discuss the experimental implications of our results.Comment: Revised version with added content (new section and figures added),
changes to wording and organizatio
Scanning Laser Acoustic Microscope Visualization of Solid Inclusions in Silicon Nitride
The Scanning Laser Acoustic Microscope (SLAM) operating at a frequency of 100 MHz is used to characterize solid inclusions in silicon nitride. Ten, seven millimeter thick discs, with 100 and 400 micron implanted inclusions are analyzed. We find that the images of a solid inclusion are characterized by a bright high transmission central zone, a well defined dark boundary and a characteristic diffraction ring pattern. These image features differentiate solid inclusions from pores and voids which may also be encountered in the samples. The images of the implanted flaws were generally found to be larger than anticipated. This can be understood in terms of the divergence of the sound due to diffraction and due to lens action of the curved boundary of the flaw. Our initial observations suggest that accurate estimates of defect size may be obtaiRable from a more complete analysis of SLAM micrographs
Differentiation of Various Flaw Types in Ceramics Using the Scanning Laser Acoustic Microscope
High frequency acoustic imaging represents a powerful technique for the nondestructive evaluation of optically opaque materials. In this report the Scanning Laser Acoustic Microscope (SLAM) is used to detect and characterize flaws in ceramics. SLAM micrographs showing typical examples of cracks, laminar flaws, porosity and solid inclusions. are presented. The various flaw types are easily differentiated on the basis of their characteristic acoustic signatures. The importance of an imaging approach to the nondestructive evaluation of ceramics is demonstrated
Analysis of Nonlinear Synchronization Dynamics of Oscillator Networks by Laplacian Spectral Methods
We analyze the synchronization dynamics of phase oscillators far from the
synchronization manifold, including the onset of synchronization on scale-free
networks with low and high clustering coefficients. We use normal coordinates
and corresponding time-averaged velocities derived from the Laplacian matrix,
which reflects the network's topology. In terms of these coordinates,
synchronization manifests itself as a contraction of the dynamics onto
progressively lower-dimensional submanifolds of phase space spanned by
Laplacian eigenvectors with lower eigenvalues. Differences between high and low
clustering networks can be correlated with features of the Laplacian spectrum.
For example, the inhibition of full synchoronization at high clustering is
associated with a group of low-lying modes that fail to lock even at strong
coupling, while the advanced partial synchronizationat low coupling noted
elsewhere is associated with high-eigenvalue modes.Comment: Revised version: References added, introduction rewritten, additional
minor changes for clarit
Further studies of the mechanism by which hydrogen enters metals during chemical and electrochemical processing
Clustering and Synchronization of Oscillator Networks
Using a recently described technique for manipulating the clustering
coefficient of a network without changing its degree distribution, we examine
the effect of clustering on the synchronization of phase oscillators on
networks with Poisson and scale-free degree distributions. For both types of
network, increased clustering hinders global synchronization as the network
splits into dynamical clusters that oscillate at different frequencies.
Surprisingly, in scale-free networks, clustering promotes the synchronization
of the most connected nodes (hubs) even though it inhibits global
synchronization. As a result, scale-free networks show an additional, advanced
transition instead of a single synchronization threshold. This cluster-enhanced
synchronization of hubs may be relevant to the brain with its scale-free and
highly clustered structure.Comment: Submitted to Phys. Rev.
A Fundamental Study of the Mechanism by which Hydrogen Enters Metals During Chemical and Electrochemical Processing
Fusion of simian immunodeficiency virus with liposomes and erythrocyte ghost membranes: Effects of lipid composition, pH and calcium
Simian immunodeficiency virus from macaques (SIV(mac)) is closely related in its structure and biological activity to human immunodeficiency virus, and is the best animal model for the acquired immunodeficiency syndrome. We investigated the kinetics of membrane fusion between SIV(mac) and phospholipid vesicles and the effects of various parameters on this process. Purified SIV(mac) was labelled with octadecyl rhodamine B chloride, and fusion was continuously monitored as the dilution of the probe in target membranes. These studies show that SIV(mac) fusion is strongly dependent upon the liposome composition. Fusion with pure cardiolipin (CL) liposomes is significantly faster than with CL/dioleoylphosphatidylcholine (DOPC) (3 : 7), phosphatidylserine (PS) or disialoganglioside (G(D1a))/DOPC (1.5 : 8.5) vesicles. SIV(mac) does not fuse appreciably with pure DOPC liposomes. Reduction of pH from 7.5 to 4.5 greatly enhances the rate of SIV(mac) fusion with CL, CL/DOPC and PS membranes, but does not affect fusion with DOPC or G(D1a)/DOPC membranes. Calcium stimulates viral fusion with CL liposomes, but not with CL/DOPC or DOPC liposomes. SIV(mac) fuses with human erythrocyte ghost membranes only slowly at reduced pH. Our results indicate that SIV(mac) can fuse with membranes lacking the known viral receptor, CD4. Although the mechanism of SIV(mac) fusion with model and biological membranes remains to be determined, the fusion activity of SIV(mac) shares similarities with other lipid-enveloped viruses such as Sendai and influenza viruses
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