1,611 research outputs found
From rods to helices: evidence of a screw-like nematic phase
Evidence of a special chiral nematic phase is provided using numerical
simulation and Onsager theory for systems of hard helical particles. This phase
appears at the high density end of the nematic phase, when helices are well
aligned, and is characterized by the C symmetry axes of the helices
spiraling around the nematic director with periodicity equal to the particle
pitch. This coupling between translational and rotational degrees of freedom
allows a more efficient packing and hence an increase of translational entropy.
Suitable order parameters and correlation functions are introduced to identify
this screw-like phase, whose main features are then studied as a function of
radius and pitch of the helical particles. Our study highlights the physical
mechanism underlying a similar ordering observed in colloidal helical flagella
[E. Barry et al. \textit{Phys. Rev. Lett.} \textbf{96}, 018305 (2006)] and
raises the question of whether it could be observed in other helical particle
systems, such as DNA, at sufficiently high densities.Comment: List of authors correcte
A numerical study of a binary Yukawa model in regimes characteristic of globular proteins in solutions
The main goal of this paper is to assess the limits of validity, in the
regime of low concentration and strong Coulomb coupling (high molecular
charges), for a simple perturbative approximation to the radial distribution
functions (RDF), based upon a low-density expansion of the potential of mean
force and proposed to describe protein-protein interactions in a recent
Small-Angle-Scattering (SAS) experimental study. A highly simplified Yukawa
(screened Coulomb) model of monomers and dimers of a charged globular protein
(-lactoglobulin) in solution is considered. We test the accuracy of the
RDF approximation, as a necessary complementary part of the previous
experimental investigation, by comparison with the fluid structure predicted by
approximate integral equations and exact Monte Carlo (MC) simulations. In the
MC calculations, an Ewald construction for Yukawa potentials has been used to
take into account the long-range part of the interactions in the weakly
screened cases. Our results confirm that the perturbative first-order
approximation is valid for this system even at strong Coulomb coupling,
provided that the screening is not too weak (i.e., for Debye length smaller
than monomer radius). A comparison of the MC results with integral equation
calculations shows that both the hypernetted-chain (HNC) and the Percus-Yevick
(PY) closures have a satisfactory behavior under these regimes, with the HNC
being superior throughout. The relevance of our findings for interpreting SAS
results is also discussed.Comment: Physical Review E, in press (2005
Real Space Renormalization Group for Langevin Dynamics in Absence of Translational Invariance
A novel exact dynamical real space renormalization group for a Langevin
equation derivable from a Euclidean Gaussian action is presented. It is
demonstrated rigorously that an algebraic temporal law holds for the Green
function on arbitrary structures of infinite extent. In the case of fractals it
is shown on specific examples that two different fixed points are found at
variance with periodic structures. Connection with growth dynamics of
interfaces is also discussed.Comment: 22 pages, RevTex 3.0, 5 figures available upon request from
[email protected], to be published in J.Stat.Phy
Circumstances of occurrence of uterine ruptures treated in a national reference maternity department in Benin from 2015 to 2019
Background: Uterine rupture is an obstetric disaster and a major concern for the obstetrician in an African environment because of the insufficient technical support. Objective of current study was to study the circumstances of occurrence of uterine ruptures.Methods: The study was carried out at the university clinic of obstetrics and gynecology of CNHU-HKM in Cotonou. This was a descriptive and cross-sectional study with retrospective collection from January 1, 2015 to December 31, 2019. We made an exhaustive recruitment of all the patients treated in the department for uterine rupture during the study period. The study variables were socio-demographic and clinical characteristics. Data confidentiality and the anonymity of women were respected.Results: The study involved 85 cases of uterine rupture. Patients were relatively young with a mean age of 30±15.02 years. Women profile was that of populations with unfavorable socio-economic conditions. The circumstances of discovery were multiparity equal to or greater than 4 (43.5%), scarred uterus (32.9%), non-use of partogram (97.6%), osseous dystocia (10.9%) and fetal dystocia with fetal macrosomia (21.2%) and dystocic presentation (15.3%).Conclusions: This study identified epidemiological and clinical characteristics related to the circumstances of known uterine ruptures occurrence. A preventive and anticipatory oriented approach can reduce the frequency of that obstetric tragedy, the adequate management of which is often uncertain in Benin
A corresponding states approach to Small-Angle-Scattering for polydisperse ionic colloidal fluids
Approximate scattering functions for polydisperse ionic colloidal fluids are
obtained by a corresponding states approach. This assumes that all pair
correlation functions of a polydisperse fluid are
conformal to those of an appropriate monodisperse binary fluid (reference
system) and can be generated from them by scaling transformations. The
correspondence law extends to ionic fluids a {\it scaling approximation} (SA)
successfully proposed for nonionic colloids in a recent paper. For the
primitive model of charged hard spheres in a continuum solvent, the partial
structure factors of the monodisperse binary reference system are evaluated by
solving the Orstein-Zernike (OZ) integral equations coupled with an approximate
closure. The SA is first tested within the mean spherical approximation (MSA)
closure, which allows analytical solutions. The results are found in good
overall agreement with exact MSA predictions up to relevant polidispersity. The
SA is shown to be an improvement over the ``decoupling approximation'' extended
to the ionic case. The simplicity of the SA scheme allows its application also
when the OZ equations can be solved only numerically. An example is then given
by using the hypernetted chain (HNC) closure. Shortcomings of the SA approach,
its possible use in the analysis of experimental scattering data and other
related points are also briefly addressed.Comment: 29 pages, 7 postscript figures (included), Latex 3.0, uses aps.sty,
to appear in Phys. Rev. E (1999
Diffusion and Trapping on a one-dimensional lattice
The properties of a particle diffusing on a one-dimensional lattice where at
each site a random barrier and a random trap act simultaneously on the particle
are investigated by numerical and analytical techniques. The combined effect of
disorder and traps yields a decreasing survival probability with broad
distribution (log-normal). Exact enumerations, effective-medium approximation
and spectral analysis are employed. This one-dimensional model shows rather
rich behaviours which were previously believed to exist only in higher
dimensionality. The possibility of a trapping-dominated super universal class
is suggested.Comment: 20 pages, Revtex 3.0, 13 figures in compressed format using uufiles
command, to appear in Phys. Rev. E, for an hard copy or problems e-mail to:
[email protected]
Fast parameter inference in a biomechanical model of the left ventricle by using statistical emulation
A central problem in biomechanical studies of personalized human left ventricular modelling is estimating the material properties and biophysical parameters from in vivo clinical measurements in a timeframe that is suitable for use within a clinic. Understanding these properties can provide insight into heart function or dysfunction and help to inform personalized medicine. However, finding a solution to the differential equations which mathematically describe the kinematics and dynamics of the myocardium through numerical integration can be computationally expensive. To circumvent this issue, we use the concept of emulation to infer the myocardial properties of a healthy volunteer in a viable clinical timeframe by using in vivo magnetic resonance image data. Emulation methods avoid computationally expensive simulations from the left ventricular model by replacing the biomechanical model, which is defined in terms of explicit partial differential equations, with a surrogate model inferred from simulations generated before the arrival of a patient, vastly improving computational efficiency at the clinic. We compare and contrast two emulation strategies: emulation of the computational model outputs and emulation of the loss between the observed patient data and the computational model outputs. These strategies are tested with two interpolation methods, as well as two loss functions. The best combination of methods is found by comparing the accuracy of parameter inference on simulated data for each combination. This combination, using the output emulation method, with local Gaussian process interpolation and the Euclidean loss function, provides accurate parameter inference in both simulated and clinical data, with a reduction in the computational cost of about three orders of magnitude compared with numerical integration of the differential equations by using finite element discretization techniques
Effect of Polydispersity and Anisotropy in Colloidal and Protein Solutions: an Integral Equation Approach
Application of integral equation theory to complex fluids is reviewed, with
particular emphasis to the effects of polydispersity and anisotropy on their
structural and thermodynamic properties. Both analytical and numerical
solutions of integral equations are discussed within the context of a set of
minimal potential models that have been widely used in the literature. While
other popular theoretical tools, such as numerical simulations and density
functional theory, are superior for quantitative and accurate predictions, we
argue that integral equation theory still provides, as in simple fluids, an
invaluable technique that is able to capture the main essential features of a
complex system, at a much lower computational cost. In addition, it can provide
a detailed description of the angular dependence in arbitrary frame, unlike
numerical simulations where this information is frequently hampered by
insufficient statistics. Applications to colloidal mixtures, globular proteins
and patchy colloids are discussed, within a unified framework.Comment: 17 pages, 7 figures, to appear in Interdiscip. Sci. Comput. Life Sci.
(2011), special issue dedicated to Prof. Lesser Blu
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