3,175 research outputs found
Fracture and Friction: Stick-Slip Motion
We discuss the stick-slip motion of an elastic block sliding along a rigid
substrate. We argue that for a given external shear stress this system shows a
discontinuous nonequilibrium transition from a uniform stick state to uniform
sliding at some critical stress which is nothing but the Griffith threshold for
crack propagation. An inhomogeneous mode of sliding occurs, when the driving
velocity is prescribed instead of the external stress. A transition to
homogeneous sliding occurs at a critical velocity, which is related to the
critical stress. We solve the elastic problem for a steady-state motion of a
periodic stick-slip pattern and derive equations of motion for the tip and
resticking end of the slip pulses. In the slip regions we use the linear
viscous friction law and do not assume any intrinsic instabilities even at
small sliding velocities. We find that, as in many other pattern forming
system, the steady-state analysis itself does not select uniquely all the
internal parameters of the pattern, especially the primary wavelength. Using
some plausible analogy to first order phase transitions we discuss a ``soft''
selection mechanism. This allows to estimate internal parameters such as crack
velocities, primary wavelength and relative fraction of the slip phase as
function of the driving velocity. The relevance of our results to recent
experiments is discussed.Comment: 12 pages, 7 figure
Classical nonlinear response of a chaotic system: Langevin dynamics and spectral decomposition
We consider the classical response of a strongly chaotic Hamiltonian system.
The spectrum of such a system consists of discrete complex Ruelle-Pollicott
(RP) resonances which manifest themselves in the behavior of the correlation
and response functions. We interpret the RP resonances as the eigenstates and
eigenvalues of the Fokker-Planck operator obtained by adding an infinitesimal
noise term to the first-order Liouville operator. We demonstrate how the
deterministic expression for the linear response is reproduced in the limit of
vanishing noise. For the second-order response we establish an equivalence of
the spectral decomposition with infinitesimal noise and the long-time
asymptotic expansion for the deterministic case.Comment: 16 pages, 1 figur
Energetics of Vesicle Fusion Intermediates: Comparison of Calculations with Observed Effects of Osmotic and Curvature Stresses
We reported previously the effects of both osmotic and curvature stress on fusion between poly(ethylene glycol)-aggregated vesicles. In this article, we analyze the energetics of fusion of vesicles of different curvature, paying particular attention to the effects of osmotic stress on small, highly curved vesicles of 26 nm diameter, composed of lipids with negative intrinsic curvature. Our calculations show that high positive curvature of the outer monolayer âchargesâ these vesicles with excess bending energy, which then releases during stalk expansion (increase of the stalk radius, rs) and thus âdrivesâ fusion. Calculations based on the known mechanical properties of lipid assemblies suggest that the free energy of âvoidâ formation as well as membrane-bending free energy dominate the evolution of a stalk to an extended transmembrane contact. The free-energy profile of stalk expansion (free energy versus rs) clearly shows the presence of two metastable intermediates (intermediate 1 at rs âŒ0 â 1.0 nm and intermediate 2 at rs âŒ2.5 â 3.0 nm). Applying osmotic gradients of ±5 atm, when assuming a fixed trans-bilayer lipid mass distribution, did not significantly change the free-energy profile. However, inclusion in the model of an additional degree of freedom, the ability of lipids to move into and out of the âvoidâ, made the free-energy profile strongly dependent on the osmotic gradient. Vesicle expansion increased the energy barrier between intermediates by âŒ4 kT and the absolute value of the barrier by âŒ7 kT, whereas compression decreased it by nearly the same extent. Since these calculations, which are based on the stalk hypothesis, correctly predict the effects of both membrane curvature and osmotic stress, they support the stalk hypothesis for the mechanism of membrane fusion and suggest that both forms of stress alter the final stages, rather than the initial step, of the fusion process, as previously suggested
Quantum creep and variable range hopping of one-dimensional interacting electrons
The variable range hopping results for noninteracting electrons of Mott and
Shklovskii are generalized to 1D disordered charge density waves and Luttinger
liquids using an instanton approach. Following a recent paper by Nattermann,
Giamarchi and Le Doussal [Phys. Rev. Lett. {\bf 91}, 56603 (2003)] we calculate
the quantum creep of charges at zero temperature and the linear conductivity at
finite temperatures for these systems. The hopping conductivity for the short
range interacting electrons acquires the same form as for noninteracting
particles if the one-particle density of states is replaced by the
compressibility. In the present paper we extend the calculation to dissipative
systems and give a discussion of the physics after the particles materialize
behind the tunneling barrier. It turns out that dissipation is crucial for
tunneling to happen. Contrary to pure systems the new metastable state does not
propagate through the system but is restricted to a region of the size of the
tunneling region. This corresponds to the hopping of an integer number of
charges over a finite distance. A global current results only if tunneling
events fill the whole sample. We argue that rare events of extra low tunneling
probability are not relevant for realistic systems of finite length. Finally we
show that an additional Coulomb interaction only leads to small logarithmic
corrections.Comment: 15 pages, 3 figures; references adde
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
The AMS-RICH velocity and charge reconstruction
The AMS detector, to be installed on the International Space Station,
includes a Ring Imaging Cerenkov detector with two different radiators, silica
aerogel (n=1.05) and sodium fluoride (n=1.334). This detector is designed to
provide very precise measurements of velocity and electric charge in a wide
range of cosmic nuclei energies and atomic numbers. The detector geometry, in
particular the presence of a reflector for acceptance purposes, leads to
complex Cerenkov patterns detected in a pixelized photomultiplier matrix. The
results of different reconstruction methods applied to test beam data as well
as to simulated samples are presented. To ensure nominal performances
throughout the flight, several detector parameters have to be carefully
monitored. The algorithms developed to fulfill these requirements are
presented. The velocity and charge measurements provided by the RICH detector
endow the AMS spectrometer with precise particle identification capabilities in
a wide energy range. The expected performances on light isotope separation are
discussed.Comment: Contribution to the ICRC07, Merida, Mexico (2007); Presenter: F.
Bara
The RICH detector of the AMS-02 experiment: status and physics prospects
The Alpha Magnetic Spectrometer (AMS), whose final version AMS-02 is to be
installed on the International Space Station (ISS) for at least 3 years, is a
detector designed to measure charged cosmic ray spectra with energies up to the
TeV region and with high energy photon detection capability up to a few hundred
GeV. It is equipped with several subsystems, one of which is a proximity
focusing RICH detector with a dual radiator (aerogel+NaF) that provides
reliable measurements for particle velocity and charge. The assembly and
testing of the AMS RICH is currently being finished and the full AMS detector
is expected to be ready by the end of 2008. The RICH detector of AMS-02 is
presented. Physics prospects are briefly discussed.Comment: 5 pages. Contribution to the 10th ICATPP Conference on Astroparticle,
Particle, Space Physics, Detectors and Medical Physics Applications (Como
2007). Presenter: Rui Pereir
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