2,492 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 . With respect to both the
cases of passive colloids with and AP with , the AP with finite
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 , where is the AP's re-orientation
time and the width characterising the surface's smoothness (
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
Infinite impulse response modal filtering in visible adaptive optics
Diffraction limited resolution adaptive optics (AO) correction in visible
wavelengths requires a high performance control. In this paper we investigate
infinite impulse response filters that optimize the wavefront correction: we
tested these algorithms through full numerical simulations of a
single-conjugate AO system comprising an adaptive secondary mirror with 1127
actuators and a pyramid wavefront sensor (WFS). The actual practicability of
the algorithms depends on both robustness and knowledge of the real system:
errors in the system model may even worsen the performance. In particular we
checked the robustness of the algorithms in different conditions, proving that
the proposed method can reject both disturbance and calibration errors
Lattice models for granular-like velocity fields: Finite-size effects
Long-range spatial correlations in the velocity and energy fields of a
granular fluid are discussed in the framework of a 1d lattice model. The
dynamics of the velocity field occurs through nearest-neighbour inelastic
collisions that conserve momentum but dissipate energy. A set of equations for
the fluctuating hydrodynamics of the velocity and energy mesoscopic fields give
a first approximation for (i) the velocity structure factor and (ii) the
finite-size correction to the Haff law, both in the homogeneous cooling regime.
At a more refined level, we have derived the equations for the two-site
velocity correlations and the total energy fluctuations. First, we seek a
perturbative solution thereof, in powers of the inverse of system size. On the
one hand, when scaled with the granular temperature, the velocity correlations
tend to a stationary value in the long time limit. On the other hand, the
scaled standard deviation of the total energy diverges, that is, the system
shows multiscaling. Second, we find an exact solution for the velocity
correlations in terms of the spectrum of eigenvalues of a certain matrix. The
results of numerical simulations of the microscopic model confirm our
theoretical results, including the above described multiscaling phenomenon
Bipolar Nickel-hydrogen Batteries for Aerospace Applications
A bipolar nickel-hydrogen battery which effectively addresses all key requirements for a spacecraft power system, including long-term reliability and low mass, is discussed. The design of this battery is discussed in the context of system requirements and nickel-hydrogen battery technology in general. To achieve the ultimate goal of an aerospace application of a bipolar Ni-H2 battery several objectives must be met in the design and development of the system. These objectives include: maximization of reliability and life; high specific energy and energy density; reasonable cost of manufacture, test, and integration; and ease in scaling for growth in power requirements. These basic objectives translate into a number of specific design requirements, which are discussed
Single prazosin infusion in prelimbic cortex Fosters extinction of amphetamine-induced conditioned place preference
Exposure to drug-associated cues to induce extinction is a useful strategy to contrast cue-induced drug seeking. Norepinephrine (NE) transmission in medial prefrontal cortex has a role in the acquisition and extinction of conditioned place preference induced by amphetamine. We have reported recently that NE in prelimbic cortex delays extinction of amphetamine-induced conditioned place preference (CPP). A potential involvement of α1-adrenergic receptors in the extinction of appetitive conditioned response has been also suggested, although their role in prelimbic cortex has not been yet fully investigated. Here, we investigated the effects of the α1-adrenergic receptor antagonist prazosin infusion in the prelimbic cortex of C57BL/6J mice on expression and extinction of amphetamine-induced CPP. Acute prelimbic prazosin did not affect expression of amphetamine-induced CPP on the day of infusion, while in subsequent days it produced a clear-cut advance of extinction of preference for the compartment previously paired with amphetamine (Conditioned stimulus, CS). Moreover, prazosin-treated mice that had extinguished CS preference showed increased mRNA expression of brain-derived neurotrophic factor (BDNF) and post-synaptic density 95 (PSD-95) in the nucleus accumbens shell or core, respectively, thus suggesting that prelimbic α1-adrenergic receptor blockade triggers neural adaptations in subcortical areas that could contribute to the extinction of cue-induced drug-seeking behavior. These results show that the pharmacological blockade of α1-adrenergic receptors in prelimbic cortex by a single infusion is able to induce extinction of amphetamine-induced CPP long before control (vehicle) animals, an effect depending on contingent exposure to retrieval, since if infused far from or after reactivation it did not affect preference. Moreover, they suggest strongly that the behavioral effects depend on post-treatment neuroplasticity changes in corticolimbic network, triggered by a possible “priming” effect of prazosin, and point
to a potential therapeutic power of the antagonist for maladaptive memories
Continuum description of finite-size particles advected by external flows. The effect of collisions
The equation of the density field of an assembly of macroscopic particles
advected by a hydrodynamic flow is derived from the microscopic description of
the system. This equation allows to recognize the role and the relative
importance of the different microscopic processes implicit in the model: the
driving of the external flow, the inertia of the particles, and the collisions
among them.
The validity of the density description is confirmed by comparisons of
numerical studies of the continuum equation with Direct Simulation Monte Carlo
(DSMC) simulations of hard disks advected by a chaotic flow. We show that the
collisions have two competing roles: a dispersing-like effect and a clustering
effect (even for elastic collisions). An unexpected feature is also observed in
the system: the presence of collisions can reverse the effect of inertia, so
that grains with lower inertia are more clusterized.Comment: Final (strongly modified) version accepted in PRE; 6 pages, 3 figure
Kohdista: An efficient method to index and query possible Rmap alignments : Algorithms for Molecular Biology
Background: Genome-wide optical maps are ordered high-resolution restriction maps that give the position of occurrence of restriction cut sites corresponding to one or more restriction enzymes. These genome-wide optical maps are assembled using an overlap-layout-consensus approach using raw optical map data, which are referred to as Rmaps. Due to the high error-rate of Rmap data, finding the overlap between Rmaps remains challenging. Results: We present Kohdista, which is an index-based algorithm for finding pairwise alignments between single molecule maps (Rmaps). The novelty of our approach is the formulation of the alignment problem as automaton path matching, and the application of modern index-based data structures. In particular, we combine the use of the Generalized Compressed Suffix Array (GCSA) index with the wavelet tree in order to build Kohdista. We validate Kohdista on simulated E. coli data, showing the approach successfully finds alignments between Rmaps simulated from overlapping genomic regions. Conclusion: we demonstrate Kohdista is the only method that is capable of finding a significant number of high quality pairwise Rmap alignments for large eukaryote organisms in reasonable time. © 2019 The Author(s).Peer reviewe
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
Non-equilibrium and information: the role of cross-correlations
We discuss the relevance of information contained in cross-correlations among
different degrees of freedom, which is crucial in non-equilibrium systems. In
particular we consider a stochastic system where two degrees of freedom
and - in contact with two different thermostats - are coupled together.
The production of entropy and the violation of equilibrium
fluctuation-dissipation theorem (FDT) are both related to the cross-correlation
between and . Information about such cross-correlation may be lost
when single-variable reduced models, for , are considered. Two different
procedures are typically applied: (a) one totally ignores the coupling with
; (b) one models the effect of as an average memory effect,
obtaining a generalized Langevin equation. In case (a) discrepancies between
the system and the model appear both in entropy production and linear response;
the latter can be exploited to define effective temperatures, but those are
meaningful only when time-scales are well separated. In case (b) linear
response of the model well reproduces that of the system; however the loss of
information is reflected in a loss of entropy production. When only linear
forces are present, such a reduction is dramatic and makes the average entropy
production vanish, posing problems in interpreting FDT violations.Comment: 30 pages, 4 figures, 4 appendixe
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