2,089 research outputs found
The scientific merit of Carpobrotus mellei L. based on antimicrobial activity and chemical profiling
Species of the genus Carpobrotus are similar in appearance and have been used for medicinal purposes over many generations. Carpobrotus mellei is endemic to the south-Western Cape of South Africa, and also used for various ailments. To date no scientific validation and information has been reported on C. mellei. This study investigated the antimicrobial potential of C. mellei against Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans and Mycobacterium smegmatis, and determined the minimum inhibitory concentrations (MIC) by two-fold serial dilution. C. mellei showed antimicrobial activity against S. aureus and M. smegmatis in the disc diffusion method. Eight chemical compounds showed clear zones of inhibition in the bioautograms, seven against S. aureus and three compounds were active against M. smegmatis. The ethyl acetate extracts have MIC values of 7.5 mg/ml and 15 mg/ml against S. aureus and M. smegmatis, respectively. Phytochemical tests indicated the presence of flavonoids, hydrolysable tannins, phytosterols and aromatic acids. High performance liquid chromatography (HPLC) showed a species-specific spectrum at a wavelength of 280nm. The results confirm that C. mellei has scientific merit, and can substitute one of the other Carpobrotus species for antimicrobial usage within the south-Western Cape of South Afric
Activity driven fluctuations in living cells
We propose a model for the dynamics of a probe embedded in a living cell,
where both thermal fluctuations and nonequilibrium activity coexist. The model
is based on a confining harmonic potential describing the elastic cytoskeletal
matrix, which undergoes random active hops as a result of the nonequilibrium
rearrangements within the cell. We describe the probe's statistics and we bring
forth quantities affected by the nonequilibrium activity. We find an excellent
agreement between the predictions of our model and experimental results for
tracers inside living cells. Finally, we exploit our model to arrive at
quantitative predictions for the parameters characterizing nonequilibrium
activity, such as the typical time scale of the activity and the amplitude of
the active fluctuations.Comment: 6 pages, 4 figure
Study of effects of fuel properties in turbine-powered business aircraft
Increased interest in research and technology concerning aviation turbine fuels and their properties was prompted by recent changes in the supply and demand situation of these fuels. The most obvious change is the rapid increase in fuel price. For commercial airplanes, fuel costs now approach 50 percent of the direct operating costs. In addition, there were occasional local supply disruptions and gradual shifts in delivered values of certain fuel properties. Dwindling petroleum reserves and the politically sensitive nature of the major world suppliers make the continuation of these trends likely. A summary of the principal findings, and conclusions are presented. Much of the material, especially the tables and graphs, is considered in greater detail later. The economic analysis and examination of operational considerations are described. Because some of the assumptions on which the economic analysis is founded are not easily verified, the sensitivity of the analysis to alternates for these assumptions is examined. The data base on which the analyses are founded is defined in a set of appendices
Monitoring spatially heterogeneous dynamics in a drying colloidal thin film
We report on a new type of experiment that enables us to monitor spatially
and temporally heterogeneous dynamic properties in complex fluids. Our approach
is based on the analysis of near-field speckles produced by light diffusely
reflected from the superficial volume of a strongly scattering medium. By
periodic modulation of an incident speckle beam we obtain pixel-wise ensemble
averages of the structure function coefficient, a measure of the dynamic
activity. To illustrate the application of our approach we follow the different
stages in the drying process of a colloidal thin film. We show that we can
access ensemble averaged dynamic properties on length scales as small as ten
micrometers over the full field of view.Comment: To appear in Soft Material
Nonlinear elasticity of stiff biopolymers connected by flexible linkers
Networks of the biopolymer actin, cross-linked by the compliant protein filamin, form soft gels. They can, however, withstand large shear stresses due to their pronounced nonlinear elastic behavior. The nonlinear elasticity can be controlled by varying the number of cross-links per actin filament. We propose and test a model of rigid filaments decorated by multiple flexible linkers that is in quantitative agreement with experiment. This allows us to estimate loads on individual cross-links, which we find to be less than 10 pN. © 2009 The American Physical Society
Valley Splitting Theory of SiGe/Si/SiGe Quantum Wells
We present an effective mass theory for SiGe/Si/SiGe quantum wells, with an
emphasis on calculating the valley splitting. The theory introduces a valley
coupling parameter, , which encapsulates the physics of the quantum well
interface. The new effective mass parameter is computed by means of a tight
binding theory. The resulting formalism provides rather simple analytical
results for several geometries of interest, including a finite square well, a
quantum well in an electric field, and a modulation doped two-dimensional
electron gas. Of particular importance is the problem of a quantum well in a
magnetic field, grown on a miscut substrate. The latter may pose a numerical
challenge for atomistic techniques like tight-binding, because of its
two-dimensional nature. In the effective mass theory, however, the results are
straightforward and analytical. We compare our effective mass results with
those of the tight binding theory, obtaining excellent agreement.Comment: 13 pages, 7 figures. Version submitted to PR
Grain Boundary Scars and Spherical Crystallography
We describe experimental investigations of the structure of two-dimensional
spherical crystals. The crystals, formed by beads self-assembled on water
droplets in oil, serve as model systems for exploring very general theories
about the minimum energy configurations of particles with arbitrary repulsive
interactions on curved surfaces. Above a critical system size we find that
crystals develop distinctive high-angle grain boundaries, or scars, not found
in planar crystals. The number of excess defects in a scar is shown to grow
linearly with the dimensionless system size. The observed slope is expected to
be universal, independent of the microscopic potential.Comment: 4 pages, 3 eps figs (high quality images available from Mark Bowick
Valley splitting of Si/SiGe heterostructures in tilted magnetic fields
We have investigated the valley splitting of two-dimensional electrons in
high quality Si/SiGe heterostructures under tilted magnetic fields.
For all the samples in our study, the valley splitting at filling factor
() is significantly different before and after the
coincidence angle, at which energy levels cross at the Fermi level. On both
sides of the coincidence, a linear density dependence of on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. We argue that screening of the Coulomb
interaction from the low-lying filled levels, which also explains the observed
spin-dependent resistivity, is responsible for the large difference of
before and after the coincidence.Comment: REVTEX 4 pages, 4 figure
Resolving long-range spatial correlations in jammed colloidal systems using photon correlation imaging
We introduce a new dynamic light scattering method, termed photon correlation
imaging, which enables us to resolve the dynamics of soft matter in space and
time. We demonstrate photon correlation imaging by investigating the slow
dynamics of a quasi two-dimensional coarsening foam made of highly packed,
deformable bubbles and a rigid gel network formed by dilute, attractive
colloidal particles. We find the dynamics of both systems to be determined by
intermittent rearrangement events. For the foam, the rearrangements extend over
a few bubbles, but a small dynamical correlation is observed up to macroscopic
length scales. For the gel, dynamical correlations extend up to the system
size. These results indicate that dynamical correlations can be extremely
long-ranged in jammed systems and point to the key role of mechanical
properties in determining their nature.Comment: Published version (Phys. Rev. Lett. 102, 085702 (2009)) The Dynamical
Activity Mapsprovided as Supplementary Online Material are also available on
http://w3.lcvn.univ-montp2.fr/~lucacip/dam/movies.ht
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