420 research outputs found
Physics of Rheologically-Enhanced Propulsion: Different Strokes in Generalized Stokes
Shear-thinning is an important rheological property of many biological
fluids, such as mucus, whereby the apparent viscosity of the fluid decreases
with shear. Certain microscopic swimmers have been shown to progress more
rapidly through shear-thinning fluids, but is this behavior generic to all
microscopic swimmers, and what are the physics through which shear-thinning
rheology affects a swimmer's propulsion? We examine swimmers employing
prescribed stroke kinematics in two-dimensional, inertialess Carreau fluid:
shear-thinning "Generalized Stokes" flow. Swimmers are modeled, using the
method of femlets, by a set of immersed, regularized forces. The equations
governing the fluid dynamics are then discretized over a body-fitted mesh and
solved with the finite element method. We analyze the locomotion of three
distinct classes of microswimmer: (1) conceptual swimmers comprising sliding
spheres employing both one- and two-dimensional strokes, (2) slip-velocity
envelope models of ciliates commonly referred to as "squirmers" and (3)
monoflagellate pushers, such as sperm. We find that morphologically identical
swimmers with different strokes may swim either faster or slower in
shear-thinning fluids than in Newtonian fluids. We explain this kinematic
sensitivity by considering differences in the viscosity of the fluid
surrounding propulsive and payload elements of the swimmer, and using this
insight suggest two reciprocal sliding sphere swimmers which violate Purcell's
Scallop theorem in shear-thinning fluids. We also show that an increased flow
decay rate arising from shear-thinning rheology is associated with a reduction
in the swimming speed of slip-velocity squirmers. For sperm-like swimmers, a
gradient of thick to thin fluid along the flagellum alters the force it exerts
upon the fluid, flattening trajectories and increasing instantaneous swimming
speed.Comment: 22 pages, 28 figure
Metamorphism of CO and CO-like chondrites and comparisons with type 3 ordinary chondrites
In order to explore their metamorphic history, thermoluminescence data have been obtained for 10 CO or CO-related chondrites from the Antarctic. Six have TL properties indicating low to intermediate levels of metamorphism, while Lewis Cliff 85332 and three paired meteorites from MacAlpine Hills (87300,87301 and 88107) have unusual TL properties similar to those of the very primitive Colony and Allan Hills A77307 CO-related chondrites. Cathodoluminescence photomosaics of nine well-studied CO chondrites are also presented and compared with similar data for the type 3 ordinary chondrites in which CL properties vary systematically with metamorphism. It is concluded that the CO chondrites, like the ordinary chondrites, form a metamorphic sequence and may be subdivided in an analogous manner using TL, CL and other petrographic and compositional data. Definitions for CO chondrites of the petrologic types 3.0-3.9 are proposed. However, it is stressed that the thermal history of the CO and ordinary chondrites is quite different, the range of equilibration for the CO chondrites is similar to the ordinary chondrites, but the former have not experienced temperatures above those experienced by type 3.5 ordinary chondrites (probably around 600℃). Presumably the CO chondrites spent longer times at lower temperatures. A CL photomosaic of Murchison is also presented, which has two features in common with the type 3.0-3.1 CO and ordinary chondrites; type I chondrules whose mesostases produce yellow CL (due to an unidentified but highly metamorphism-sensitive phase) and fine-grained matrix with red CL due to forsterite. Haloes of matrix material around chondrules and other objects in Murchison are thought to be due to aqueous destruction of those objects, and Fezoning in olivines in chondrules with broad haloes is also throught to be due to aqueous processes
The packing of two species of polygons on the square lattice
We decorate the square lattice with two species of polygons under the
constraint that every lattice edge is covered by only one polygon and every
vertex is visited by both types of polygons. We end up with a 24 vertex model
which is known in the literature as the fully packed double loop model. In the
particular case in which the fugacities of the polygons are the same, the model
admits an exact solution. The solution is obtained using coordinate Bethe
ansatz and provides a closed expression for the free energy. In particular we
find the free energy of the four colorings model and the double Hamiltonian
walk and recover the known entropy of the Ice model. When both fugacities are
set equal to two the model undergoes an infinite order phase transition.Comment: 21 pages, 4 figure
Drying of complex suspensions
We investigate the 3D structure and drying dynamics of complex mixtures of
emulsion droplets and colloidal particles, using confocal microscopy. Air
invades and rapidly collapses large emulsion droplets, forcing their contents
into the surrounding porous particle pack at a rate proportional to the square
of the droplet radius. By contrast, small droplets do not collapse, but remain
intact and are merely deformed. A simple model coupling the Laplace pressure to
Darcy's law correctly estimates both the threshold radius separating these two
behaviors, and the rate of large-droplet evacuation. Finally, we use these
systems to make novel hierarchical structures.Comment: 4 pages, 4 figure
Vortex Formation by Interference of Multiple Trapped Bose-Einstein Condensates
We report observations of vortex formation as a result of merging together
multiple Rb Bose-Einstein condensates (BECs) in a confining potential.
In this experiment, a trapping potential is partitioned into three sections by
a barrier, enabling the simultaneous formation of three independent,
uncorrelated condensates. The three condensates then merge together into one
BEC, either by removal of the barrier, or during the final stages of
evaporative cooling if the barrier energy is low enough; both processes can
naturally produce vortices within the trapped BEC. We interpret the vortex
formation mechanism as originating in interference between the initially
independent condensates, with indeterminate relative phases between the three
initial condensates and the condensate merging rate playing critical roles in
the probability of observing vortices in the final, single BEC.Comment: 5 pages, 3 figure
Radio Astronomy
Contains report on one research project.National Aeronautics and Space Administration (Grant NGL 22-009-016)National Science Foundation (Grant GP-14854)National Science Foundation (Grant GP-13056
Factors affecting diet, habitat selection and breeding success of the African crowned eagle Stephanoaetus coronatus in a fragmented landscape
This study aimed to identify variables that affect habitat selection and nesting success of the African Crowned
Eagle Stephanoaetus coronatus, the largest forest raptor, in north-eastern South Africa. A preference for nesting
in the Northern Mistbelt Forest vegetation type was established and 82% of all nests were located in indigenous
trees. Nest abandonment was less common when distances to the nearest neighbour were greater. The diet of this
species was investigated by examination of prey remains beneath nests and verified by comparison with museum
specimens. In total, 156 remains were found, representing a minimum of 75 prey individuals. The diet of African
Crowned Eagles constituted almost entirely mammals (99%), which were predominantly antelopes (61%) and
monkeys (25%). It was also found that the proportion of primates in the diet correlates with latitude: populations in
equatorial latitudes have a higher proportion of primates in their diets, whereas further south antelopes are a much
more common diet component.Komatiland Forests, BirdLife South Africa, Sappi Forests, Fishwick Nurseries, Bird Life Lowveld, Barberton Bird Club and GIS Map (Nelspruit).http://www.tandfonline.com/loi/tost20hb2014Zoology and EntomologyMammal Research Institut
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
Expression of Na+/glucose co-transporter 1 (SGLT1) in the intestine of piglets weaned to different concentrations of dietary carbohydrate
Na+/glucose co-transporter 1 (SGLT1) transports dietary sugars from the lumen of the intestine into enterocytes. Regulation of this protein is essential for the provision of glucose to the body and, thus, is important for maintenance of glucose homeostasis. We have assessed expression of SGLT1 at mRNA, protein and functional levels in the intestinal tissue of 28d old piglets weaned onto isoenergetic diets with differing concentrations of digestible carbohydrate (CHO). We show that expression of SGLT1 remains constant when piglets are fed up to 40% CHO-containing diets. However, there is a significant increase in SGLT1 expression when the CHO content of the diet is>50%. Morphometric analyses indicate that the increased expression is not due to a trophic effect. It has been proposed that in rat intestine, in response to a high-CHO diet, GLUT2 (the classical basolateral membrane monosaccharide transporter) is translocated to the luminal membrane of enterocytes to absorb excess dietary glucose. We show, using immunohistochemistry and Western blotting with antibodies raised to amino acids in different epitopes of GLUT2, that under all dietary conditions, low to high CHO, GLUT2 is expressed on the basolateral membrane of pig enterocytes. Furthermore, functional studies indicate that there is no uptake of 2-deoxy-d-glucopyranoside, a specific substrate of Na+-independent glucose transporters into brush-border membrane vesicles isolated from the intestines of piglets either maintained on low- or high-CHO diets. Thus, SGLT1 is the major route for absorption of dietary sugars across the luminal membrane of swine enterocyte
Expression of Na+/glucose co-transporter 1 (SGLT1) is enhanced by supplementation of the diet of weaning piglets with artificial sweeteners
In an intensive livestock production, a shorter suckling period allows more piglets to be born. However, this practice leads to a number of disorders including nutrient malabsorption, resulting in diarrhoea, malnutrition and dehydration. A number of strategies have been proposed to overcome weaning problems. Artificial sweeteners, routinely included in piglets' diet, were thought to enhance feed palatability. However, it is shown in rodent models that when included in the diet, they enhance the expression of Na+/glucose co-transporter (SGLT1) and the capacity of the gut to absorb glucose. Here, we show that supplementation of piglets' feed with a combination of artificial sweeteners saccharin and neohesperidin dihydrochalcone enhances the expression of SGLT1 and intestinal glucose transport function. Artificial sweeteners are known to act on the intestinal sweet taste receptor T1R2/T1R3 and its partner G-protein, gustducin, to activate pathways leading to SGLT1 up-regulation. Here, we demonstrate that T1R2, T1R3 and gustducin are expressed together in the enteroendocrine cells of piglet intestine. Furthermore, gut hormones secreted by the endocrine cells in response to dietary carbohydrates, glucagon-like peptides (GLP)-1, GLP-2 and glucose-dependent insulinotrophic peptide (GIP), are co-expressed with type 1 G-protein-coupled receptors (T1R) and gustducin, indicating that L- and K-enteroendocrine cells express these taste elements. In a fewer endocrine cells, T1R are also co-expressed with serotonin. Lactisole, an inhibitor of human T1R3, had no inhibitory effect on sweetener-induced SGLT1 up-regulation in piglet intestine. A better understanding of the mechanism(s) involved in sweetener up-regulation of SGLT1 will allow the identification of nutritional targets with implications for the prevention of weaning-related malabsorptio
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