422 research outputs found
The possible role of r-modes in post-glitch relaxation of Crab
The loss of angular momentum through gravitational radiation, driven by the
excitation of r-modes, is considered in neutron stars having rotation
frequencies smaller than the associated critical frequency. We find that for
reasonable values of the initial amplitudes of such pulsation modes of the
star, being excited at the event of a glitch in a pulsar, the total post-glitch
losses correspond to a negligible fraction of the initial rise of the spin
frequency in the case of Vela and the older pulsars. However, for the Crab
pulsar the same effect would result, within a few months, in a decrease in its
spin frequency by an amount larger than its glitch-induced frequency increase.
This could provide an explanation for the peculiar behavior observed in the
post-glitch relaxations of the Crab.Comment: 9 pages, 4 figures, RevTe
Effect of Soil Moisture Evaporation Rate on Dynamic Measurement of Water Retention Curve with High-Capacity Tensiometer
This paper investigates the effect of soil moisture evaporation rate on the soil water retention curve (SWRC) of clays obtained using high-capacity tensiometer (HCT) technique and following the continuous drying (dynamic) method. SWRC measurements, with and without soil moisture evaporation rate control, were carried out on reconstituted London clay specimens using 12 performance-improved HCTs recently developed at the University of Warwick. Furthermore, the HCTsâ performance in terms of the maximum attainable suction (sâââ) and maximum measurement duration (tâââ) was evaluated. Moreover, the suitability of a curve fitting-based model, available in the literature, for attaining the entire retention curve (beyond the capacity of HCTs) was evaluated. The SWRCs for tests with controlled evaporation rate were found to be generally exhibiting higher suctions at a given water content, hence inducing air-entry values that were on average 16% higher than those obtained from tests without evaporation rate control. It was also found that for suctions beyond 2 MPa, the curve fitting-based model predictions of data obtained from tests with controlled evaporation rate exhibit significantly lower suctions at a given water content than those without evaporation rate control, suggesting that such curve fitting correlations should be used with caution
Geological and Geotechnical Characteristics of London Clay from the Isle of Sheppey
The paper describes a series of experimental testing on natural stiff London clay samples retrieved from the New Hook Farm in the Isle of Sheppey, UK. The experimental program includes determination of macroscopic and microscopic properties, chemical compositions, Atterberg limits, permeability, and compressibility parameters in both intact and reconstituted states. The paper integrates the earlier studies, the new findings, and the commercial investigation results with the aim to extend the current knowledge of the geological and geotechnical characteristics of this stiff clay from the east of the London basin. Some comparisons are also made with shallow depth London clay from Heathrow Terminal 5 site
An experimental study of the initial volumetric strain rate effect on the creep behaviour of reconstituted clays
Clayey soils tend to undergo continuous compression with time, even after excess pore pressures have substantially dissipated. The effect of time on deformation and mechanical response of these soft soils has been the subject of numerous studies. Based on these studies, the observed time-dependent behaviour of clays is mainly related to the evolution of soil volume and strength characteristics with time, which are classified as creep and/or relaxation properties of the soil. Apart from many empirical relationships that have been proposed in the literature to capture the rheological behaviour of clays, a number of viscid constitutive relationships have also been developed which have more attractive theoretical attributes. A particular feature of these viscid models is that their creep parameters often have clear physical meaning (e.g. coefficient of secondary compression, Cα). Sometimes with these models, a parameter referred to as initial/reference volumetric strain rate, vÌâ has also been alluded as a model parameter. However, unlike Cα, the determination of vÌâ and its variations with stress level is not properly documented in the literature. In an attempt to better understand vÌâ, this paper presents an experimental investigation of the reference volumetric strain rate in reconstituted clay specimens. A long-term triaxial creep test, at different shear stress levels and different strain rates, was performed on clay specimen whereby the volumetric strain rate was measured. The obtained results indicated the stress-level dependency and non-linear variation of vÌâ with time
Cavitation in high-capacity tensiometers: effect of water reservoir surface roughness
High-capacity tensiometers (HCTs) are sensors made to measure negative pore water pressure (suction) directly. In
this paper, a new approach is proposed to expand the range and duration of suction measurements for a newly
designed HCT. A new technique is employed to reduce significantly the roughness of the diaphragmâs surface on the
water reservoir side in order to minimise the possibility of gas nuclei development and the subsequent early
cavitation at the waterâdiaphragm interface. The procedures employed for the design, fabrication, saturation and
calibration of the new tensiometers are explained in detail. Furthermore, the performance of the developed HCTs is
examined based on a series of experiments carried out on a number of unsaturated clay specimens. An improvement
in maximum sustainable suction in the range of 120â150% of their nominal capacity was obtained from different
surface treatment methods. Moreover, the results show an improvement of up to 177% for the long-term stability
of measurements, compared to the developed ordinary HCTs with untreated diaphragms
Thermal conductivity of anisotropic spin - 1/2 two leg ladder:Green's function approach
We study the thermal transport of a spin-1/2 two leg antiferromagnetic ladder
in the direction of legs. The possible effect of spin-orbit coupling and
crystalline electric field are investigated in terms of anisotropies in the
Heisenberg interactions on both leg and rung couplings. The original spin
ladder is mapped to a bosonic model via a bond-operator transformation where an
infinite hard-core repulsion is imposed to constrain one boson occupation per
site. The Green's function approach is applied to obtain the energy spectrum of
quasi-particle excitations responsible for thermal transport. The thermal
conductivity is found to be monotonically decreasing with temperature due to
increased scattering among triplet excitations at higher temperatures. A tiny
dependence of thermal transport on the anisotropy in the leg direction at low
temperatures is observed in contrast to the strong one on the anisotropy along
the rung direction, due to the direct effect of the triplet density. Our
results reach asymptotically the ballistic regime of the spin - 1/2 Heisenberg
chain and compare favorably well with exact diagonalization data
An experimental investigation of the independent effect of suction and degree of saturation on very small-strain stiffness of unsaturated sand
The paper presents an experimental investigation of very small strain stiffness of unsaturated sand. A triaxial test apparatus was equipped with bender elements and compression discs in order to assess the stiffness at very small strains by measuring the velocity of propagation of shear and compression waves through an unsaturated sample. The negative water column method was adopted to apply suction at the base of the sample. The experiments were designed to investigate the independent effect of suction and degree of saturation on the wave propagation velocities. This was achieved by testing the sand sample on both the drying and wetting path
A continuous model for microtubule dynamics with catastrophe, rescue and nucleation processes
Microtubules are a major component of the cytoskeleton distinguished by
highly dynamic behavior both in vitro and in vivo. We propose a general
mathematical model that accounts for the growth, catastrophe, rescue and
nucleation processes in the polymerization of microtubules from tubulin dimers.
Our model is an extension of various mathematical models developed earlier
formulated in order to capture and unify the various aspects of tubulin
polymerization including the dynamic instability, growth of microtubules to
saturation, time-localized periods of nucleation and depolymerization as well
as synchronized oscillations exhibited by microtubules under various
experimental conditions. Our model, while attempting to use a minimal number of
adjustable parameters, covers a broad range of behaviors and has predictive
features discussed in the paper. We have analyzed the resultant behaviors of
the microtubules changing each of the parameter values at a time and observing
the emergence of various dynamical regimes.Comment: 25 pages, 12 figure
Modeling the Effects of Drug Binding on the Dynamic Instability of Microtubules
We propose a stochastic model that accounts for the growth, catastrophe and
rescue processes of steady state microtubules assembled from MAP-free tubulin.
Both experimentally and theoretically we study the perturbation of microtubule
dynamic instability by S-methyl-D-DM1, a synthetic derivative of the
microtubule-targeted agent maytansine and a potential anticancer agent. We find
that to be an effective suppressor of microtubule dynamics a drug must
primarily suppress the loss of GDP tubulin from the microtubule tip.Comment: 17 pages, 11 figures, to appear in Phys. Bio
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