272 research outputs found
Regulation of Growth of Fibroblasts
Present knowledge on regulation of fibroblast growth is based on in vitro culture of fibroblasts from different sources. The research has focused on 2 problems: identification of the signal that reaches the fibroblast from outside and tells it to grow and identification of the metabolic reactions inside the cell that commit it to initiate DNA synthesis after the signal arrives. Although the signal and the metabolic reactions have not yet been clearly identified, and the relationship between in vivo conditions and the result of these in vitro studies still has to be determined, the large body of data collected so far and the steadily growing information concerning these problems suggest a complex interrelation between cellular environment and metabolic processes involved in growth regulation
Lifetimes of Confined Acoustic Phonons in Ultra-Thin Silicon Membranes
We study the relaxation of coherent acoustic phonon modes with frequencies up
to 500 GHz in ultra-thin free-standing silicon membranes. Using an ultrafast
pump-probe technique of asynchronous optical sampling, we observe that the
decay time of the first-order dilatational mode decreases significantly from
\sim 4.7 ns to 5 ps with decreasing membrane thickness from \sim 194 to 8 nm.
The experimental results are compared with theories considering both intrinsic
phonon-phonon interactions and extrinsic surface roughness scattering including
a wavelength-dependent specularity. Our results provide insight to understand
some of the limits of nanomechanical resonators and thermal transport in
nanostructures
Lateral migration of a 2D vesicle in unbounded Poiseuille flow
The migration of a suspended vesicle in an unbounded Poiseuille flow is
investigated numerically in the low Reynolds number limit. We consider the
situation without viscosity contrast between the interior of the vesicle and
the exterior. Using the boundary integral method we solve the corresponding
hydrodynamic flow equations and track explicitly the vesicle dynamics in two
dimensions. We find that the interplay between the nonlinear character of the
Poiseuille flow and the vesicle deformation causes a cross-streamline migration
of vesicles towards the center of the Poiseuille flow. This is in a marked
contrast with a result [L.G. Leal, Ann. Rev. Fluid Mech. 12,
435(1980)]according to which the droplet moves away from the center (provided
there is no viscosity contrast between the internal and the external fluids).
The migration velocity is found to increase with the local capillary number
(defined by the time scale of the vesicle relaxation towards its equilibrium
shape times the local shear rate), but reaches a plateau above a certain value
of the capillary number. This plateau value increases with the curvature of the
parabolic flow profile. We present scaling laws for the migration velocity.Comment: 11 pages with 4 figure
Heap Formation in Granular Media
Using molecular dynamics (MD) simulations, we find the formation of heaps in
a system of granular particles contained in a box with oscillating bottom and
fixed sidewalls. The simulation includes the effect of static friction, which
is found to be crucial in maintaining a stable heap. We also find another
mechanism for heap formation in systems under constant vertical shear. In both
systems, heaps are formed due to a net downward shear by the sidewalls. We
discuss the origin of net downward shear for the vibration induced heap.Comment: 11 pages, 4 figures available upon request, Plain TeX, HLRZ-101/9
Continuous Avalanche Segregation of Granular Mixtures in Thin Rotating Drums
We study segregation of granular mixtures in the continuous avalanche regime
(for frequencies above ~ 1 rpm) in thin rotating drums using a continuum theory
for surface flows of grains. The theory predicts profiles in agreement with
experiments only when we consider a flux dependent velocity of flowing grains.
We find the segregation of species of different size and surface properties,
with the smallest and roughest grains being found preferentially at the center
of the drum. For a wide difference between the species we find a complete
segregation in agreement with experiments. In addition, we predict a transition
to a smooth segregation regime - with an power-law decay of the concentrations
as a function of radial coordinate - as the size ratio between the grains is
decreased towards one.Comment: 4 pages, 4 figures, http://polymer.bu.edu/~hmaks
Effects of long-term soluble vs. insoluble dietary fiber intake on high-fat diet-induced obesity in C57BL/6J mice
Although most of the proposed beneficial effects of fiber consumption have been attributed to viscous and gel-forming properties of soluble fiber, it is mainly insoluble cereal fiber and whole grains that are strongly associated with reduced diabetes risk in prospective cohort studies, indicating that other unknown mechanisms are likely to be involved.
We performed a long-term study investigating potential protective effects of adding soluble guar fiber (10% w/w) vs. insoluble cereal fiber (10% w/w) to an isoenergetic and macronutrient matched high-fat diet in obesity-prone C57BL/6J mice. After 45 weeks, mice fed soluble vs. insoluble fiber showed both significantly increased body weight (41.8±3.0 vs. 33.6±1.5 g, P=.03) and elevated markers of insulin resistance. In mice fed soluble fiber, energy loss via the feces was significantly lower and colonic fermentation with production of short chain fatty acids (SCFA) was markedly increased. Gene expression analysis in white adipose tissue showed significantly increased levels of the fatty acid target G-protein coupled receptor-40 in soluble fiber-fed mice. Liver gene expression in the insoluble fiber group showed a pattern consistent with increased fatty acid oxidation. The present results show that soluble vs insoluble dietary fiber added to a high-fat, Western-style diet differently affected body weight and estimates of insulin sensitivity in obesity-prone mice. Soluble fiber intake with increased SCFA production significantly contributed to digested energy, thereby potentially outweighing the well known short-term beneficial effects of soluble fiber consumption
Angle of Repose and Angle of Marginal Stability: Molecular Dyanmics of Granular Particles
We present an implementation of realistic static friction in molecular
dynamics (MD) simulations of granular particles. In our model, to break
contacts between two particles, one has to apply a finite amount of force,
determined by the Coulomb criterion. Using a two dimensional model, we show
that piles generated by avalanches have a {\it finite} angle of repose
(finite slopes). Furthermore, these piles are stable under tilting
by an angle smaller than a non-zero tilting angle , showing that
is different from the angle of marginal stability ,
which is the maximum angle of stable piles. These measured angles are compared
to a theoretical approximation. We also measure by continuously
adding particles on the top of a stable pile.Comment: 14 pages, Plain Te
Localized and Cellular Patterns in a Vibrated Granular Layer
We propose a phenomenological model for pattern formation in a vertically
vibrated layer of granular material. This model exhibits a variety of stable
cellular patterns including standing rolls and squares as well as localized
objects (oscillons and worms), similar to recent experimental
observations(Umbanhowar et al., 1996). The model is an amplitude equation for
the parametrical instability coupled to the mass conservation law. The
structure and dynamics of the solutions resemble closely the properties of
localized and cellular patterns observed in the experiments.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Antioxidant Machinery Differs between Melanic and Light Nestlings of Two Polymorphic Raptors
Colour polymorphism results from the expression of multiallelic genes generating phenotypes with very distinctive colourations. Most colour polymorphisms are due to differences in the type or amount of melanins present in each morph, which also differ in several behavioural, morphometric and physiological attributes. Melanin-based colour morphs could also differ in the levels of glutathione (GSH), a key intracellular antioxidant, because of the role of this molecule in melanogenesis. As GSH inhibits the synthesis of eumelanin (i.e. the darkest melanin form), individuals of darker morphs are expected to have lower GSH levels than those of lighter morphs. We tested this prediction in nestlings of two polymorphic raptors, the booted eagle Hieraaetus pennatus and the Eleonora's falcon Falco eleonorae, both of which occur in two morphs differing in the extent of eumelanic plumage. As expected, melanic booted eagle nestlings had lower blood GSH levels than light morph eagle nestlings. In the Eleonora's falcon, however, melanic nestlings only had lower GSH levels after controlling for the levels of other antioxidants. We also found that melanic female eagle nestlings had higher levels of antioxidants other than GSH and were in better body condition than light female eagle nestlings. These findings suggest an adaptive response of melanic nestlings to compensate for reduced GSH levels. Nevertheless, these associations were not found in falcons, indicating species-specific particularities in antioxidant machinery. Our results are consistent with previous work revealing the importance of GSH on the expression of melanic characters that show continuous variation, and suggest that this pathway also applies to discrete colour morphs. We suggest that the need to maintain low GSH levels for eumelanogenesis in dark morph individuals may represent a physiological constraint that helps regulate the evolution and maintenance of polymorphisms
Size Segregation and Convection of Granular Mixtures Almost Completely Packed in the Rotating Thin Box
Size segregation of granular mixtures which are almost completely packed in a
rotating drum is discussed with an effective simulation and a brief analysis.
Instead of a 3D drum, we simulate 2D rotating thin box which is almost
completely packed with granular mixtures. The phase inversion of radially
segregated pattern which was found in a 3D experiment are qualitatively
reproduced with this simulation, and a brief analysis is followed. Moreover in
our simulation, a global convection appears after radial segregation pattern is
formed, and this convection induces axially segregated pattern.Comment: 9 pages, 5 figures, PACS number(s): 45.70.-n, 45.70.M
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