504 research outputs found
The field theoretic derivation of the contact value theorem in planar geometries and its modification by the Casimir effect
The contact value theorem for Coulomb gases in planar or film-like geometries
is derived using a Hamiltonian field theoretic representation of the system.
The case where the film is enclosed by a material of different dielectric
constant to that of the film is shown to contain an additional Casimir-like
term which is generated by fluctuations of the electric potential about its
mean-field value.Comment: Link between Sine-Gordon and Coulomb gas pressures via subtraction of
self interaction terms included. Discussion of results within Debye-Huckel
approximation included. Added reference
Survival and ice nucleation activity of bacteria as aerosols in a cloud simulation chamber
The residence time of bacterial cells in the atmosphere is predictable by numerical models. However, estimations of their aerial dispersion as living entities are limited by a lack of information concerning survival rates and behavior in relation to atmospheric water. Here we investigate the viability and ice nucleation (IN) activity of typical atmospheric ice nucleation active bacteria (Pseudomonas syringae and P. fluorescens) when airborne in a cloud simulation chamber (AIDA, Karlsruhe, Germany). Cell suspensions were sprayed into the chamber and aerosol samples were collected by impingement at designated times over a total duration of up to 18 h, and at some occasions after dissipation of a cloud formed by depressurization. Aerosol concentration was monitored simultaneously by online instruments. The cultivability of airborne cells decreased exponentially over time with a half-life time of 250 ±30 min (about 3.5 to 4.5 h). In contrast, IN activity remained unchanged for several hours after aerosolization, demonstrating that IN activity was maintained after cell death. Interestingly, the relative abundance of IN active cells still airborne in the chamber was strongly decreased after cloud formation and dissipation. This illustrates the preferential precipitation of IN active cells by wet processes. Our results indicate that from 10ⶠcells aerosolized from a surface, one would survive the average duration of its atmospheric journey estimated at 3.4 days. Statistically, this corresponds to the emission of 1 cell that achieves dissemination every ~33 min mÂŻÂČ of cultivated crops fields, a strong source of airborne bacteria. Based on the observed survival rates, depending on wind speed, the trajectory endpoint could be situated several hundreds to thousands of kilometers from the emission source. These results should improve the representation of the aerial dissemination of bacteria in numeric models
ABEMUS: platform specific and data informed detection of somatic SNVs in cfDNA
MOTIVATION: The use of liquid biopsies for cancer patients enables the non-invasive tracking of treatment response and tumor dynamics through single or serial blood drawn tests. Next generation sequencing assays allow for the simultaneous interrogation of extended sets of somatic single nucleotide variants (SNVs) in circulating cell free DNA (cfDNA), a mixture of DNA molecules originating both from normal and tumor tissue cells. However, low circulating tumor DNA (ctDNA) fractions together with sequencing background noise and potential tumor heterogeneity challenge the ability to confidently call SNVs. RESULTS: We present a computational methodology, called Adaptive Base Error Model in Ultra-deep Sequencing data (ABEMUS), which combines platform-specific genetic knowledge and empirical signal to readily detect and quantify somatic SNVs in cfDNA. We tested the capability of our method to analyze data generated using different platforms with distinct sequencing error properties and we compared ABEMUS performances with other popular SNV callers on both synthetic and real cancer patients sequencing data. Results show that ABEMUS performs better in most of the tested conditions proving its reliability in calling low variant allele frequencies somatic SNVs in low ctDNA levels plasma samples. AVAILABILITY: ABEMUS is cross-platform and can be installed as R package. The source code is maintained on Github at http://github.com/cibiobcg/abemus and it is also available at CRAN official R repository. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online
Steric Effects in Electrolytes: A Modified Poisson-Boltzmann Equation
The adsorption of large ions from solution to a charged surface is
investigated theoretically. A generalized Poisson--Boltzmann equation, which
takes into account the finite size of the ions is presented. We obtain
analytical expressions for the electrostatic potential and ion concentrations
at the surface, leading to a modified Grahame equation. At high surface charge
densities the ionic concentration saturates to its maximum value. Our results
are in agreement with recent experiments.Comment: 4 pages, 2 figure
Infrared and optical polarimetry around the low-mass star-forming region NGC 1333 IRAS 4A
We performed J- and R-band linear polarimetry with the 4.2 m William Herschel
Telescope at the Observatorio del Roque de los Muchachos and with the 1.6 m
telescope at the Observat\'orio do Pico dos Dias, respectively, to derive the
magnetic field geometry of the diffuse molecular cloud surrounding the embedded
protostellar system NGC 1333 IRAS 4A. We obtained interstellar polarization
data for about two dozen stars. The distribution of polarization position
angles has low dispersion and suggests the existence of an ordered magnetic
field component at physical scales larger than the protostar. Some of the
observed stars present intrinsic polarization and evidence of being young
stellar objects. The estimated mean orientation of the interstellar magnetic
field as derived from these data is almost perpendicular to the main direction
of the magnetic field associated with the dense molecular envelope around IRAS
4A. Since the distribution of the CO emission in NGC 1333 indicates that the
diffuse molecular gas has a multi-layered structure, we suggest that the
observed polarization position angles are caused by the superposed projection
along the line of sight of different magnetic field components.Comment: 37 pages, 9 figures, 5 tables, accepted for publication in A
The `Friction' of Vacuum, and other Fluctuation-Induced Forces
The static Casimir effect describes an attractive force between two
conducting plates, due to quantum fluctuations of the electromagnetic (EM)
field in the intervening space. {\it Thermal fluctuations} of correlated fluids
(such as critical mixtures, super-fluids, liquid crystals, or electrolytes) are
also modified by the boundaries, resulting in finite-size corrections at
criticality, and additional forces that effect wetting and layering phenomena.
Modified fluctuations of the EM field can also account for the `van der Waals'
interaction between conducting spheres, and have analogs in the
fluctuation--induced interactions between inclusions on a membrane. We employ a
path integral formalism to study these phenomena for boundaries of arbitrary
shape. This allows us to examine the many unexpected phenomena of the dynamic
Casimir effect due to moving boundaries. With the inclusion of quantum
fluctuations, the EM vacuum behaves essentially as a complex fluid, and
modifies the motion of objects through it. In particular, from the mechanical
response function of the EM vacuum, we extract a plethora of interesting
results, the most notable being: (i) The effective mass of a plate depends on
its shape, and becomes anisotropic. (ii) There is dissipation and damping of
the motion, again dependent upon shape and direction of motion, due to emission
of photons. (iii) There is a continuous spectrum of resonant cavity modes that
can be excited by the motion of the (neutral) boundaries.Comment: RevTex, 2 ps figures included. The presentation is completely
revised, and new sections are adde
Effective Interactions and Volume Energies in Charged Colloids: Linear Response Theory
Interparticle interactions in charge-stabilized colloidal suspensions, of
arbitrary salt concentration, are described at the level of effective
interactions in an equivalent one-component system. Integrating out from the
partition function the degrees of freedom of all microions, and assuming linear
response to the macroion charges, general expressions are obtained for both an
effective electrostatic pair interaction and an associated microion volume
energy. For macroions with hard-sphere cores, the effective interaction is of
the DLVO screened-Coulomb form, but with a modified screening constant that
incorporates excluded volume effects. The volume energy -- a natural
consequence of the one-component reduction -- contributes to the total free
energy and can significantly influence thermodynamic properties in the limit of
low-salt concentration. As illustrations, the osmotic pressure and bulk modulus
are computed and compared with recent experimental measurements for deionized
suspensions. For macroions of sufficient charge and concentration, it is shown
that the counterions can act to soften or destabilize colloidal crystals.Comment: 14 pages, including 3 figure
Effective forces in colloidal mixtures: from depletion attraction to accumulation repulsion
Computer simulations and theory are used to systematically investigate how
the effective force between two big colloidal spheres in a sea of small spheres
depends on the basic (big-small and small-small) interactions. The latter are
modeled as hard-core pair potentials with a Yukawa tail which can be both
repulsive or attractive. For a repulsive small-small interaction, the effective
force follows the trends as predicted by a mapping onto an effective
non-additive hard-core mixture: both a depletion attraction and an accumulation
repulsion caused by small spheres adsorbing onto the big ones can be obtained
depending on the sign of the big-small interaction. For repulsive big-small
interactions, the effect of adding a small-small attraction also follows the
trends predicted by the mapping. But a more subtle ``repulsion through
attraction'' effect arises when both big-small and small-small attractions
occur: upon increasing the strength of the small-small interaction, the
effective potential becomes more repulsive. We have further tested several
theoretical methods against our computer simulations: The superposition
approximation works best for an added big-small repulsion, and breaks down for
a strong big-small attraction, while density functional theory is very accurate
for any big-small interaction when the small particles are pure hard-spheres.
The theoretical methods perform most poorly for small-small attractions.Comment: submitted to PRE; New version includes an important quantitative
correction to several of the simulations. The main conclusions remain
unchanged thoug
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