11,074 research outputs found

### Self-propulsion against a moving membrane: enhanced accumulation and drag force

Self-propulsion (SP) is a main feature of active particles (AP), such as
bacteria or biological micromotors, distinguishing them from passive colloids.
A renowned consequence of SP is accumulation at static interfaces, even in the
absence of hydrodynamic interactions. Here we address the role of SP in the
interaction between AP and a moving semipermeable membrane. In particular, we
implement a model of noninteracting AP in a channel crossed by a partially
penetrable wall, moving at a constant velocity $c$. With respect to both the
cases of passive colloids with $c>0$ and AP with $c=0$, the AP with finite $c$
show enhancement of accumulation in front of the obstacle and experience a
largely increased drag force. This effect is understood in terms of an
effective potential localised at the interface between particles and membrane,
of height proportional to $c\tau/\xi$, where $\tau$ is the AP's re-orientation
time and $\xi$ the width characterising the surface's smoothness ($\xi\to 0$
for hard core obstacles). An approximate analytical scheme is able to reproduce
the observed density profiles and the measured drag force, in very good
agreement with numerical simulations. The effects discussed here can be
exploited for automatic selection and filtering of AP with desired parameters.Comment: 13 pages, 3 figure

### Metadynamic sampling of the free energy landscapes of proteins coupled with a Monte Carlo algorithm

Metadynamics is a powerful computational tool to obtain the free energy
landscape of complex systems. The Monte Carlo algorithm has proven useful to
calculate thermodynamic quantities associated with simplified models of
proteins, and thus to gain an ever-increasing understanding on the general
principles underlying the mechanism of protein folding. We show that it is
possible to couple metadynamics and Monte Carlo algorithms to obtain the free
energy of model proteins in a way which is computationally very economical.Comment: Submitted to Gen

### Which is the temperature of granular systems? A mean field model of free cooling inelastic mixtures

We consider a mean field model describing the free cooling process of a two
component granular mixture, a generalization of so called Maxwell model. The
cooling is viewed as an ordering process and the scaling behavior is attributed
to the presence of an attractive fixed point at $v=0$ for the dynamics. By
means of asymptotic analysis of the Boltzmann equation and of numerical
simulations we get the following results: 1)we establish the existence of two
different partial granular temperatures, one for each component, which violates
the Zeroth Law of Thermodynamics; 2) we obtain the scaling form of the two
distribution functions; 3) we prove the existence of a continuous spectrum of
exponents characterizing the inverse-power law decay of the tails of the
velocity, which generalizes the previously reported value 4 for the pure model;
4) we find that the exponents depend on the composition, masses and restitution
coefficients of the mixture; 5) we also remark that the reported distributions
represent a dynamical realization of those predicted by the Non Extensive
Statistical Mechanics, in spite of the fact that ours stem from a purely
dynamical approach.Comment: 23 pages, 9 figures. submitted for publicatio

### Science leadership for tomorrow: The role of schools of public affairs and universities in meeting needs of public science agencies

Recommendations and requirements for the preparation of personnel with some scientific or technological background to enter fields of public policy and administration are reported. University efforts to provide science administration graduate programs are outlined and increased cooperation between government and university resources is outlined

### Driven low density granular mixtures

We study the steady state properties of a 2D granular mixture in the presence
of energy driving by employing simple analytical estimates and Direct
Simulation Monte Carlo. We adopt two different driving mechanisms: a) a
homogeneous heat bath with friction and b) a vibrating boundary (thermal or
harmonic) in the presence of gravity. The main findings are: the appearance of
two different granular temperatures, one for each species; the existence of
overpopulated tails in the velocity distribution functions and of non trivial
spatial correlations indicating the spontaneous formation of cluster
aggregates. In the case of a fluid subject to gravity and to a vibrating
boundary, both densities and temperatures display non uniform profiles along
the direction normal to the wall, in particular the temperature profiles are
different for the two species while the temperature ratio is almost constant
with the height. Finally, we obtained the velocity distributions at different
heights and verified the non gaussianity of the resulting distributions.Comment: 19 pages, 12 figures, submitted for publicatio

### Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes

The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure

### Steady state properties of a mean field model of driven inelastic mixtures

We investigate a Maxwell model of inelastic granular mixture under the
influence of a stochastic driving and obtain its steady state properties in the
context of classical kinetic theory. The model is studied analytically by
computing the moments up to the eighth order and approximating the
distributions by means of a Sonine polynomial expansion method. The main
findings concern the existence of two different granular temperatures, one for
each species, and the characterization of the distribution functions, whose
tails are in general more populated than those of an elastic system. These
analytical results are tested against Monte Carlo numerical simulations of the
model and are in general in good agreement. The simulations, however, reveal
the presence of pronounced non-gaussian tails in the case of an infinite
temperature bath, which are not well reproduced by the Sonine method.Comment: 23 pages, 10 figures, submitted for publicatio

### Collision and fusion of counterpropagating micron-sized optical beams in non-uniformly biased photorefractive crystals

We theoretically investigate collision of optical beams travelling in
opposite directions through a centrosymmetric photorefractive crystal biased by
a spatially non-uniform voltage. We analytically predict the fusion of
counterpropagating solitons in conditions in which the applied voltage is
rapidly modulated along the propagation axis, so that self-bending is
suppressed by the "restoring symmetry" mechanism. Moreover, when the applied
voltage is slowly modulated, we predict that the modified self-bending allows
conditions in which the two beams fuse together, forming a curved light-channel
splice.Comment: 12 page

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