47,552 research outputs found
Rubber friction on wet and dry road surfaces: the sealing effect
Rubber friction on wet rough substrates at low velocities is typically 20-30%
smaller than for the corresponding dry surfaces. We show that this cannot be
due to hydrodynamics and propose a novel explanation based on a sealing effect
exerted by rubber on substrate "pools" filled with water. Water effectively
smoothens the substrate, reducing the major friction contribution due to
induced viscoelastic deformations of the rubber by surface asperities. The
theory is illustrated with applications related to tire-road friction.Comment: Format Revtex 4; 8 pages, 11 figures (no color); Published on Phys.
Rev. B (http://link.aps.org/abstract/PRB/v71/e035428); previous work on the
same topic: cond-mat/041204
Shakura-Sunyaev Disk Can Smoothly Match Advection-Dominated Accretion Flow
We use the standard Runge-Kutta method to solve the set of basic equations
describing black hole accretion flows composed of two-temperature plasma. We do
not invoke any extra energy transport mechanism such as thermal conduction and
do not specify any ad hoc outer boundary condition for the advection-dominated
accretion flow (ADAF) solution. We find that in the case of high viscosity and
non-zero radiative cooling, the ADAF solution can have an asymptotic approach
to the Shakura-Sunyaev disk (SSD) solution, and the SSD-ADAF transition radius
is close to the central black hole. Our results further prove the mechanism of
thermal instability-triggered SSD-ADAF transition suggested previously by
Takeuchi & Mineshige and Gu & Lu.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter
Object orientation without extending Z
The good news of this paper is that without extending Z,
we can elegantly specify object-oriented systems, including encapsulation,
inheritance and subtype polymorphism (dynamic dispatch). The
bad news is that this specification style is rather different to normal
Z specifications, more abstract and axiomatic, which means that it is
not so well supported by current Z tools such as animators. It also enforces
behavioural subtyping, unlike most object-oriented programming
languages. This paper explains the proposed style, with examples, and
discusses its advantages and disadvantages
Two-temperature coronal flow above a thin disk
We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu,
& Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including
different temperatures in ions and electrons as well as Compton cooling. We
found that the mass evaporation rate and hence the fraction of accretion energy
released in the corona depend strongly on the rate of incoming mass flow from
outer edge of the disk, a larger rate leading to more Compton cooling, less
efficient evaporation and a weaker corona. We also found a strong dependence on
the viscosity, higher viscosity leading to an enhanced mass flow in the corona
and therefore more evaporation of gas from the disk below. If we take accretion
rates in units of the Eddington rate our results become independent on the mass
of the central black hole. The model predicts weaker contributions to the hard
X-rays for objects with higher accretion rate like narrow-line Seyfert 1
galaxies (NLS1s), in agreement with observations. For luminous active galactic
nuclei (AGN) strong Compton cooling in the innermost corona is so efficient
that a large amount of additional heating is required to maintain the corona
above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte
Coarse-Grained Modeling of Genetic Circuits as a Function of the Inherent Time Scales
From a coarse-grained perspective the motif of a self-activating species,
activating a second species which acts as its own repressor, is widely found in
biological systems, in particular in genetic systems with inherent oscillatory
behavior. Here we consider a specific realization of this motif as a genetic
circuit, in which genes are described as directly producing proteins, leaving
out the intermediate step of mRNA production. We focus on the effect that
inherent time scales on the underlying fine-grained scale can have on the
bifurcation patterns on a coarser scale in time. Time scales are set by the
binding and unbinding rates of the transcription factors to the promoter
regions of the genes. Depending on the ratio of these rates to the decay times
of the proteins, the appropriate averaging procedure for obtaining a
coarse-grained description changes and leads to sets of deterministic
equations, which differ in their bifurcation structure. In particular the
desired intermediate range of regular limit cycles fades away when the binding
rates of genes are of the same order or less than the decay time of at least
one of the proteins. Our analysis illustrates that the common topology of the
widely found motif alone does not necessarily imply universal features in the
dynamics.Comment: 29 pages, 16 figure
Accelerating Bayesian inference for evolutionary biology models.
Bayesian inference is widely used nowadays and relies largely on Markov chain Monte Carlo (MCMC) methods. Evolutionary biology has greatly benefited from the developments of MCMC methods, but the design of more complex and realistic models and the ever growing availability of novel data is pushing the limits of the current use of these methods.
We present a parallel Metropolis-Hastings (M-H) framework built with a novel combination of enhancements aimed towards parameter-rich and complex models. We show on a parameter-rich macroevolutionary model increases of the sampling speed up to 35 times with 32 processors when compared to a sequential M-H process. More importantly, our framework achieves up to a twentyfold faster convergence to estimate the posterior probability of phylogenetic trees using 32 processors when compared to the well-known software MrBayes for Bayesian inference of phylogenetic trees.
https://bitbucket.org/XavMeyer/hogan.
[email protected].
Supplementary data are available at Bioinformatics online
Targeting tumor multicellular aggregation through IGPR-1 inhibits colon cancer growth and improves chemotherapy
Adhesion to extracellular matrix (ECM) is crucially important for survival of normal epithelial cells as detachment from ECM triggers specific apoptosis known as anoikis. As tumor cells lose the requirement for anchorage to ECM, they rely on cell-cell adhesion 'multicellular aggregation' for survival. Multicellular aggregation of tumor cells also significantly determines the sensitivity of tumor cells to the cytotoxic effects of chemotherapeutics. In this report, we demonstrate that expression of immunoglobulin containing and proline-rich receptor-1 (IGPR-1) is upregulated in human primary colon cancer. Our study demonstrates that IGPR-1 promotes tumor multicellular aggregation, and interfering with its adhesive function inhibits multicellular aggregation and, increases cell death. IGPR-1 supports colon carcinoma tumor xenograft growth in mouse, and inhibiting its activity by shRNA or blocking antibody inhibits tumor growth. More importantly, IGPR-1 regulates sensitivity of tumor cells to the chemotherapeutic agent, doxorubicin/adriamycin by a mechanism that involves doxorubicin-induced AKT activation and phosphorylation of IGPR-1 at Ser220. Our findings offer novel insight into IGPR-1's role in colorectal tumor growth, tumor chemosensitivity, and as a possible novel anti-cancer target.Grant support from: R01 CA175382/CA/NCI NIH HHS/United States, R21 CA191970/CA/NCI NIH HHS/United States, and R21 CA193958/CA/NCI NIH HHS/United State
First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen
Using density-functional theory in combination with a thermodynamic formalism
we calculate the relative stability of various structural models of the polar
O-terminated (000-1)-O surface of ZnO. Model surfaces with different
concentrations of oxygen vacancies and hydrogen adatoms are considered.
Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2
gas phase we determine a phase diagram of the lowest-energy surface structures.
For a wide range of temperatures and pressures we find that hydrogen will be
adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At
high temperatures and low pressures the hydrogen can be removed and a structure
with 1/4 of the surface oxygen atoms missing becomes the most stable one. The
clean, defect-free surface can only exist in an oxygen-rich environment with a
very low hydrogen partial pressure. However, since we find that the
dissociative adsorption of molecular hydrogen and water (if also the
Zn-terminated surface is present) is energetically very preferable, it is very
unlikely that a clean, defect-free (000-1)-O surface can be observed in
experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro
Density-functional study of Cu atoms, monolayers, and coadsorbates on polar ZnO surfaces
The structure and electronic properties of single Cu atoms, copper monolayers
and thin copper films on the polar oxygen and zinc terminated surfaces of ZnO
are studied using periodic density-functional calculations. We find that the
binding energy of Cu atoms sensitively depends on how charge neutrality of the
polar surfaces is achieved. Bonding is very strong if the surfaces are
stabilized by an electronic mechanism which leads to partially filled surface
bands. As soon as the surface bands are filled (either by partial Cu coverage,
by coadsorbates, or by the formation of defects), the binding energy decreases
significantly. In this case, values very similar to those found for nonpolar
surfaces and for copper on finite ZnO clusters are obtained. Possible
implications of these observations concerning the growth mode of copper on
polar ZnO surfaces and their importance in catalysis are discussed.Comment: 6 pages with 2 postscript figures embedded. Uses REVTEX and epsf
macro
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