2,171 research outputs found
Nonlinear Criterion for the Stability of Molecular Clouds
Dynamically significant magnetic fields are routinely observed in molecular
clouds, with mass-to-flux ratio lambda = (2 pi sqrt{G}) (Sigma/B) ~ 1 (here
Sigma is the total column density and B is the field strength). It is widely
believed that ``subcritical'' clouds with lambda < 1 cannot collapse, based on
virial arguments by Mestel and Spitzer and a linear stability analysis by
Nakano and Nakamura. Here we confirm, using high resolution numerical models
that begin with a strongly supersonic velocity dispersion, that this criterion
is a fully nonlinear stability condition. All the high-resolution models with
lambda <= 0.95 form ``Spitzer sheets'' but collapse no further. All models with
lambda >= 1.02 collapse to the maximum numerically resolvable density. We also
investigate other factors determining the collapse time for supercritical
models. We show that there is a strong stochastic element in the collapse time:
models that differ only in details of their initial conditions can have
collapse times that vary by as much as a factor of 3. The collapse time cannot
be determined from just the velocity dispersion; it depends also on its
distribution. Finally, we discuss the astrophysical implications of our
results.Comment: 11 pages, 5 figure
Adhesive-based selection by a tentacle-feeding polychaete for particle size, shape and bacterial coating in silt and sand
We tested particle selection by a surface deposit-feeding, tentaculate spionid polychaete, Pseudopolydora kempi japonica Imajima and Hartman. In experiments with peroxide-cleaned, sizegraded but otherwise natural silts and sands, individual worms showed peak preference for particles 80-99 ÎŒm in diameter, compared with previous (Self and Jumars, 1988) documentation of peak preference for particles of 7 ÎŒm in experiments conducted with plastic and glass beads. These results imply that microtektites will not in general be good tracers of mixing of mineral grains of comparable size. Animals exhibited statistically significant but not marked differences in size selectivity for subrounded versus subangular grain shapes; the size preference peak was broader in subangular grains, for which orientation of the grain can alter probabilities of both contact and retention. When one size class of grains was coated with the bacterium Halomonas halodurans (ATCC 29686), animals in general showed enhanced selection (relative to controls with no food value on any size class) of that size and smaller grains, even though these smaller grains lacked food value. Greatest selection, however, generally occurred for the coated size class. Results from inclusion of glass beads in some of the experiments and from separate experiments with tentacle analogs imply that this selective capability may be largely passive and mechanical. Natural grains, due to surface texture, have more surface area for adhesive contact than do smooth glass beads so that larger grains than beads are retained. Bacteria-coated grains, in turn, appear to be picked up preferentially due to adhesion with the bacterial coating; petroleum jelly-coated microscope slides also succeed in selective retention of the size class that is bacterially coated. Substantial selection by adhesive-utilizing deposit feeders apparently can be achieved without investment in complex, time- and energy-consuming sensory systems and behaviors. Experiments with natural grains showed notably more scatter than prior experiments with glass beads, but this difference is consistent with the mechanism. For nonspherical particles, both contact and retention depend on orientation as well as size
Extracellular antifreeze protein significantly enhances the cryopreservation of cell monolayers
The cryopreservation of cells underpins many areas of biotechnology, healthcare, and fundamental science by enabling the banking and distribution of cells. Cryoprotectants are essential to prevent cold-induced damage. Here, we demonstrate that extracellular localization of antifreeze proteins can significantly enhance post-thaw recovery of mammalian cell monolayers cryopreserved using dimethyl sulfoxide, whereas they show less benefit in suspension cryopreservation. A type III antifreeze protein (AFPIII) was used as the macromolecular ice recrystallization inhibitor and its intra/extracellular locations were controlled by using Pep-1, a cell-penetrating peptide. Flow cytometry and confocal microscopy confirmed successful delivery of AFPIII. The presence of extracellular AFPIII dramatically increased post-thaw recovery in a challenging 2-D cell monolayer system using just 0.8 mg·mLâ1, from 25% to over 60%, whereas intracellularly delivered AFPIII showed less benefit. Interestingly, the antifreeze protein was less effective when used in suspension cryopreservation of the same cells, suggesting that the cryopreservation format is also crucial. These observations show that, in the discovery of macromolecular cryoprotectants, intracellular delivery of ice recrystallization inhibitors may not be a significant requirement under âslow freezingâ conditions, which will help guide the design of new biomaterials, in particular, for cell storage
Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb
The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the
Mn-based class of Heuslers and half-Heuslers that contains several conventional
and half metallic ferromagnets, shows a peculiar stability of its magnetic
order in high magnetic fields. Density functional based studies reveal an
unusual nature of its unstable (and therefore unseen) paramagnetic state, which
for one electron less (CuMnSn, for example) would be a zero gap semiconductor
(accidentally so) between two sets of very narrow, topologically separate bands
of Mn 3d character. The extremely flat Mn 3d bands result from the environment:
Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below
the Fermi level, and the other four tetrahedrally coordinated sites are empty,
leaving chemically isolated Mn 3d states. The AFM phase can be pictured
heuristically as a self-doped CuMnSb compensated semimetal
with heavy mass electrons and light mass holes, with magnetic coupling
proceeding through Kondo and/or antiKondo coupling separately through the two
carrier types. The ratio of the linear specific heat coefficient and the
calculated Fermi level density of states indicates a large mass enhancement
, or larger if a correlated band structure is taken as the
reference
Phase vortices from a Young's three-pinhole interferometer
An analysis is presented of the phase vortices generated in the far field, by
an arbitrary arrangement of three monochromatic point sources of complex
spherical waves. In contrast with the case of three interfering plane waves, in
which an infinitely-extended vortex lattice is generated, the spherical sources
generate a finite number of phase vortices. Analytical expressions for the
vortex core locations are developed and shown to have a convenient
representation in a discrete parameter space. Our analysis may be mapped onto
the case of a coherently-illuminated Young's interferometer, in which the
screen is punctured by three rather than two pinholes.Comment: 10 pages, 8 figures, REVTeX4, Submitted to Phys. Rev.
Designing a Replication Study in Kinesiology: Lessons from the Field
The submitted presentation material summarizes a project presented at the 2021 Cal Poly Virtual BEACoN Symposium. The title of the project which the presentation is based is, âTowards Equitable Communication: Explorations to Guide Knowledge Translation in Kinesiology.â The uploaded file document presents the presentation abstract, student testimony, as well as suggested citations for individual aspects of the presentation material. Please follow the social media profiles of the faculty mentor to the project, Dr. Thomas, for timely project updates. You may find related work from this lab group published to Cal Poly Digital Commons under the Kinesiology and Public Health section (see URL): https://digitalcommons.calpoly.edu/kinesp/ . Finally, a copy of the video presentation itself has been attached. Patrons are encouraged to use the file itself in their work. The video has English subtitles
Engineering cell surfaces by covalent grafting of synthetic polymers to metabolically-labeled glycans
Re-engineering mammalian cell surfaces enables modulation of their phenotype, function, and interactions with external markers and may find application in cell-based therapies. Here we use metabolic glycan labeling to install azido groups onto the cell surface, which can act as anchor points to enable rapid, simple, and robust âclickâ functionalization by the addition of a polymer bearing orthogonally reactive functionality. Using this strategy, new cell surface functionality was introduced by using telechelic polymers with fluorescence or biotin termini, demonstrating that recruitment of biomacromolecules is possible. This approach may enable the attachment of payloads and modulation of cell function and fate, as well as providing a tool to interface synthetic polymers with biological systems
Experimental study of out of equilibrium fluctuations in a colloidal suspension of Laponite using optical traps
This work is devoted to the study of displacement fluctuations of
micron-sized particles in an aging colloidal glass. We address the issue of the
validity of the fluctuation dissipation theorem (FDT) and the time evolution of
viscoelastic properties during aging of aqueous suspensions of a clay (Laponite
RG) in a colloidal glass phase. Given the conflicting results reported in the
literature for different experimental techniques, our goal is to check and
reconcile them using \emph{simultaneously} passive and active microrheology
techniques. For this purpose we measure the thermal fluctuations of micro-sized
brownian particles immersed in the colloidal glass and trapped by optical
tweezers. We find that both microrheology techniques lead to compatible results
even at low frequencies and no violation of FDT is observed. Several
interesting features concerning the statistical properties and the long time
correlations of the particles are observed during the transition
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