32 research outputs found
Yield stress and elasticity influence on surface tension measurements
We have performed surface tension measurements on carbopol gels of different
concentrations and yield stresses. Our setup, based on the force exerted by a
capillary bridge on two parallel plates, allows to measure an effective surface
tension of the complex fluid and to investigate the influence of flow history.
More precisely the effective surface tension measured after stretching the
bridge is always higher than after compressing it. The difference between the
two values is due to the existence of a yield stress in the fluid. The
experimental observations are successfully reproduced with a simple
elasto-plastic model. The shape of successive stretching-compression cycles can
be described by taking into account the yield stress and the elasticity of the
gel. We show that the surface tension of yield stress fluids is
the mean of the effective surface tension values only if the elastic modulus is
high compared to the yield stress. This work highlights that thermodynamical
quantities measurements are challenged by the fluid out-of-equilibrium state
implied by jamming, even at small scales where the shape of the bridge is
driven by surface energy. Therefore setups allowing deformation in opposite
directions are relevant for measurements on yield stress fluids.Comment: 12 pages, 16 figures in Soft Matter 201
Deformation upon impact of a concentrated suspension drop
We study the impact between a plate and a drop of non-colloidal solid
particles suspended in a Newtonian liquid, with a specific attention to the
case when the particle volume fraction, , is close to - or even exceeds -
the critical volume fraction, , at which the steady effective viscosity
of the suspension diverges. We use a specific concentration protocol together
with an accurate determination of for each drop and we measure the
deformation for different liquid viscosities, impact velocities and
particle sizes. At low volume fractions, is found to follow closely an
effective Newtonian behavior, which we determine by documenting the low
deformation limit for a highly viscous Newtonian drop and characterizing the
effective shear viscosity of our suspensions. By contrast, whereas the
effective Newtonian approach predicts that vanishes at , a
finite deformation is observed for . This finite deformation
remains controlled by the suspending liquid viscosity and increases with
increasing particle size, which suggests that the dilatancy of the particle
phase is a key factor of the dissipation process close to and above .Comment: Submitted to JF
The acidic domain of the endothelial membrane protein GPIHBP1 stabilizes lipoprotein lipase activity by preventing unfolding of its catalytic domain.
GPIHBP1 is a glycolipid-anchored membrane protein of capillary endothelial cells that binds lipoprotein lipase (LPL) within the interstitial space and shuttles it to the capillary lumen. The LPL•GPIHBP1 complex is responsible for margination of triglyceride-rich lipoproteins along capillaries and their lipolytic processing. The current work conceptualizes a model for the GPIHBP1•LPL interaction based on biophysical measurements with hydrogen-deuterium exchange/mass spectrometry, surface plasmon resonance, and zero-length cross-linking. According to this model, GPIHBP1 comprises two functionally distinct domains: (1) an intrinsically disordered acidic N-terminal domain; and (2) a folded C-terminal domain that tethers GPIHBP1 to the cell membrane by glycosylphosphatidylinositol. We demonstrate that these domains serve different roles in regulating the kinetics of LPL binding. Importantly, the acidic domain stabilizes LPL catalytic activity by mitigating the global unfolding of LPL's catalytic domain. This study provides a conceptual framework for understanding intravascular lipolysis and GPIHBP1 and LPL mutations causing familial chylomicronemia
Multi-year patterns in testosterone, cortisol and corticosterone in baleen from adult males of three whale species
© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Conservation Physiology 6 (2018): coy049, doi:10.1093/conphys/coy049.Male baleen whales have long been suspected to have annual cycles in testosterone, but due to difficulty in collecting endocrine samples, little direct evidence exists to confirm this hypothesis. Potential influences of stress or adrenal stress hormones (cortisol, corticosterone) on male reproduction have also been difficult to study. Baleen has recently been shown to accumulate steroid hormones during growth, such that a single baleen plate contains a continuous, multi-year retrospective record of the whale’s endocrine history. As a preliminary investigation into potential testosterone cyclicity in male whales and influences of stress, we determined patterns in immunoreactive testosterone, two glucocorticoids (cortisol and corticosterone), and stable-isotope (SI) ratios, across the full length of baleen plates from a bowhead whale (Balaena mysticetus), a North Atlantic right whale (Eubalaena glacialis) and a blue whale (Balaenoptera musculus), all adult males. Baleen was subsampled at 2 cm (bowhead, right) or 1 cm (blue) intervals and hormones were extracted from baleen powder with methanol, followed by quantification of all three hormones using enzyme immunoassays validated for baleen extract of these species. Baleen of all three males contained regularly spaced peaks in testosterone content, with number and spacing of testosterone peaks corresponding well to SI data and to species-specific estimates of annual baleen growth rate. Cortisol and corticosterone exhibited some peaks that co-occurred with testosterone peaks, while other glucocorticoid peaks occurred independent of testosterone peaks. The right whale had unusually high glucocorticoids during a period with a known entanglement in fishing gear and a possible disease episode; in the subsequent year, testosterone was unusually low. Further study of baleen testosterone patterns in male whales could help clarify conservation- and management-related questions such as age of sexual maturity, location and season of breeding, and the potential effect of anthropogenic and natural stressors on male testosterone cycles.This work was supported by (1) the Arizona Board of Regents Technology Research Initiative Fund; (2) the Center for Bioengineering Innovation at Northern Arizona University; (3) the Greenland Institute of Natural Resources; (4) the Woods Hole Oceanographic Institution Ocean Life Institute and (5) Fisheries and Ocean Canada’s (DFO) Priorities and Partnership Strategic Initiatives Fund and Oceans Protection Plan
Deuterated formaldehyde in rho Ophiuchi A
From mapping observations of H2CO, HDCO, and D2CO, we have determined how the
degree of deuterium fractionation changes over the central 3'x3' region of rho
Oph A. The multi-transition data of the various H2CO isotopologues, as well as
from other molecules (e.g., CH3OH and N2D+) present in the observed bands, were
analysed using both the standard type rotation diagram analysis and, in
selected cases, a more elaborate method of solving the radiative transfer for
optically thick emission. In addition to molecular column densities, the
analysis also estimates the kinetic temperature and H2 density. Toward the SM1
core in rho Oph A, the H2CO deuterium fractionation is very high. In fact, the
observed D2CO/HDCO ratio is 1.34+/-0.19, while the HDCO/H2CO ratio is
0.107+/-0.015. This is the first time, to our knowledge, that the D2CO/HDCO
abundance ratio is observed to be greater than 1. The kinetic temperature is in
the range 20-30 K in the cores of rho Oph A, and the H2 density is (6-10)x10^5
cm-3. We estimate that the total H2 column density toward the deuterium peak is
(1-4)x10^23 cm-2. As depleted gas-phase chemistry is not adequate, we suggest
that grain chemistry, possibly due to abstraction and exchange reactions along
the reaction chain H2CO -> HDCO -> D2CO, is at work to produce the very high
deuterium levels observed.Comment: 17 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Exports and Productivity: Comparable Evidence for 14 Countries
We use comparable micro level panel data for 14 countries and a set of identically specified empirical models to investigate the relationship between exports and productivity. Our overall results are in line with the big picture that is by now familiar from the literature: Exporters are more productive than non-exporters when observed and unobserved heterogeneity are controlled for, and these exporter productivity premia tend to increase with the share of exports in total sales; there is strong evidence in favour of self-selection of more productive firms into export markets, but nearly no evidence in favour of the learning-by-exporting hypothesis. We document that the exporter premia differ considerably across countries in identically specified empirical models. In a meta-analysis of our results we find that countries that are more open and have more effective government report higher productivity premia. However, the level of development per se does not appear to be an explanation for the observed cross-country differences.exports; productivity; micro data; international comparison
The initial conditions of isolated star formation - X. A suggested evolutionary diagram for prestellar cores
We propose an evolutionary path for prestellar cores on the radius-mass
diagram, which is analogous to stellar evolutionary paths on the
Hertzsprung-Russell Diagram. Using James Clerk Maxwell Telescope (JCMT)
observations of L1688 in the Ophiuchus star-forming complex, we analyse the
HCO+ (J=4\rightarrow3) spectral line profiles of prestellar cores. We find that
of the 58 cores observed, 14 show signs of infall in the form of a
blue-asymmetric double-peaked line profile. These 14 cores all lie beyond the
Jeans mass line for the region on a radius-mass plot. Furthermore another 10
cores showing tentative signs of infall, in their spectral line profile shapes,
appear on or just over the Jeans mass line. We therefore propose the manner in
which a prestellar core evolves across this diagram. We hypothesise that a core
is formed in the low-mass, low-radius region of the plot. It then accretes
quasistatically, increasing in both mass and radius. When it crosses the limit
of gravitational instability it begins to collapse, decreasing in radius,
towards the region of the diagram where protostellar cores are seen.Comment: 42 pages, 11 figures (+58 in appendix), Accepted for publication in
MNRA
Star formation in Perseus - V. Outflows detected by HARP
Molecular outflows provide an alternative method of identifying protostellar
cores, complementary to recent mid-infrared studies. Continuing our studies of
Perseus, we investigate whether all Spitzer-identified protostars, and
particularly those with low luminosities, drive outflows, and if any new
protostellar cores (perhaps harbouring low-mass sources) can be identified via
their outflows alone. We have used the heterodyne array receiver HARP on JCMT
to make deep 12CO 3-2 maps of submm cores in Perseus, extending and deepening
our earlier study with RxB and bringing the total number of SCUBA cores studied
up to 83. Our survey includes 23/25 of the Dunham et al. (2008) Spitzer
low-luminosity objects believed to be embedded protostars, including three
VeLLOs. All but one of the cores identified as harbouring embedded YSOs have
outflows, confirming outflow detections as a good method for identifying
protostars. We detect outflows from 20 Spitzer low-luminosity objects. We do
not conclusively detect any outflows from IR-quiet cores, though confusion in
clustered regions such as NGC1333 makes it impossible to identify all the
individual driving sources. This similarity in detection rates despite the
difference in search methods and detection limits suggests either that the
sample of protostars in Perseus is now complete, or that the existence of an
outflow contributes to the Spitzer detectability, perhaps through the
contribution of shocked H2 emission in the IRAC bands. For five of the
low-luminosity sources, there is no protostellar envelope detected at 350
microns and the Spitzer emission is entirely due to shocks. Additionally, we
detect the outflow from IRAS 03282+3035 at 850 microns with SCUBA due to CO
line contamination in the continuum passband.Comment: 25 pages, 12 figures, to be published in A&
Mouillage de fluides à seuil : ponts capillaires et étalement de gouttes
Wetting phenomena and yield-stress fluids rheology are subfields of soft matter physics where big understanding steps have been made during the last centuries. In addition, these two fields have very important potential implications for industry, which contributes to their dynamism. But their combination, the wetting of yield-stress fluids, has received little interest until the very last years, although it is a situation that happens all the time. Indeed, yield-stress fluids gather nearly all the fluids encountered in food industry, cosmetics, building industry, oil and gas industry… and wetting properties are crucial when processing or using the fluids, as many processes involve interfaces with air or a solid surface.In this thesis, I consider the following questions: how is the apparent surface tension affected by yield stress? How does the yield stress influence the wetting dynamics, classically described by Tanner’s law? Why can the final contact angle of a sessile drop of yield-stress fluid not be predicted by Young-Dupré’s theory?I performed experiments with a model yield-stress fluid called carbopol. The first experiment consisted in measuring the adhesion force of a capillary bridge and comparing it to the case of simple fluids. The main results show the importance of the deformation history and of the fluid elasticity. The second main experiment concerned spreading of drops on a hydrophilic surface. I studied the short-time dynamics and the long-time dynamics, as well as the final contact angle. The first regime is controlled by viscoelasticity, whereas the final state is determined by the yield stressLes phénomènes de mouillage et la rhéologie des fluides à seuil sont deux domaines de la physique des matériaux mous dans lesquels de grandes avancées ont été faites lors des derniers siècles. De plus ces questions sont d'une grande importance au niveau des applications industrielles, ce qui contribue à leur dynamisme. En revanche, le mouillage des fluides à seuil a été peu étudié, alors que c'est une situation fréquente. En effet, presque tous les fluides rencontrés dans l'industrie et la vie quotidienne sont des fluides à seuil. D'autre part, la connaissance des propriétés de mouillage est cruciale lors de leur manipulation car la plupart des processus font intervenir des interfaces.Dans ma thèse, je m'intéresse aux questions suivantes : comment la tension de surface apparente est-elle affectée par le seuil ? Comment le seuil influence-t-il la dynamique du mouillage, habituellement décrite par la loi de Tanner ? Pourquoi l'angle de contact d'une goutte de fluide à seuil n'est-il pas prédit par la loi d'Young-Dupré ?J'ai réalisé des expériences sur un fluide à seuil modèle appelé carbopol. La première expérience a consisté à mesurer la force d'adhésion d'un pont capillaire, qui a été comparée au cas des fluides simples. Les résultats ont montré l'importance de l'histoire de la déformation et de l'élasticité du fluide. La seconde expérience a porté sur l'étalement de gouttes sur une surface hydrophile. J'ai étudié la dynamique d'étalement, ainsi que l'angle de contact final. Alors que la dynamique est contrôlée par la viscoélasticité, l'état final est déterminé par le seui
Lehmann rotatory power: a new concept in cholesteric liquid crystals
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