arXiv subjects

Nicolas Louvet

Publications and source records attributed to Nicolas Louvet.

5 recordsLinked to original sources

arXiv2025

All in one: holographic microscopy unveils bacterial transport from single cells to population

Prior to pioneer surface adhesion, bacteria have to navigate in flows, often in confined environments. While much is known about their individual swimming dynamics, our understanding of their transport properties at the population level remains limited. This is primarily due to the experimental challenges associated with tracking, in three dimensions and under flow, a large sample of these microorganisms. Here we investigate, through fast digital holographic microscopy (DHM), a suspension of Shewanella oneidensis MR-1 bacteria in a confined Poiseuille flow. Based on the analysis of several thousand Lagrangian trajectories, we first demonstrate the ability of DHM to discriminate between the fraction of bacteria that adhere to walls, those that are only convected by the flow, and those for which motility helps them migrate through the channel and cross streamlines. Focusing on the motile part of the population, we report new experimental results concerning the spatial and orientational distributions across the confinement direction. In the central part of the flow, migration and shear trapping are observed and well described by existing kinetic models. Close to the walls, averaging over the entire motile population clearly shows an accumulation layer but with no net orientational order, which contradicts models that neglect hydrodynamic interactions with solid surfaces. We think that DHM coupled with the analysis of large-scale bacterial populations will help to understand their transfer mechanisms from the bulk to surfaces.

physics.bio-ph↗

arXiv2016

Shear-banding and Taylor-Couette instability in thixotropic yield stress fluids

In the present work, we study the flow of thixotropic yield stress fluids between two concentric cylinders. In order to take into account the thixotropy, the constitutive relation uses a structural parameter which is driven by a kinetic equation. Here, the Houska's model is considered. Depending on the breakdown rate of the structural parameter, localization or shear-banding are observed. We show that for fragile structures, a shear-banding flow may be observed although for stronger structures, only localisation of the flow is observed such as in Bingham fluids. Physical explanations of the shear-banding discussed by several authors in the literature highlight that the shear-banding may be associated with a discontinuity into the structure of the material and a non-monotonic evolution of the stress according to the constitutive relation with the strain rate. Solving numerically the flow, we show that such a rheological model based on the existence of a structural parameter is able to predict shear-banding. Moreover, the consequences of the thixotropy on the linear stability of the azimuthal flow is studied in a large range of parameters. Although the thixotropy allows shear banding for the base flow, it does not modify fundamentaly the stability of the Couette flow compared to a simple yield stress fluid. The apparent shear-thinning behaviour depends on the thixotropic parameters of the fluid and the results about the onset of the Taylor vortices in shear-thinning fluids are retrieved. Nevertheless, the shear-banding modifies the stratification of the viscosity in the flowing zone such that the critical conditions are mainly driven by the width of the flowing region.

physics.flu-dyn↗

arXiv2010

The Howe-Moore property for real and p-adic groups

We consider in this paper a relative version of the Howe-Moore Property, about vanishing at infinity of coefficients of unitary representations. We characterize this property in terms of ergodic measure-preserving actions. We also characterize, for linear Lie groups or p-adic Lie groups, the pairs with the relative Howe-Moore Property with respect to a closed, normal subgroup. This involves, in one direction, structural results on locally compact groups all of whose proper closed characteristic subgroups are compact, and, in the other direction, some results about the vanishing at infinity of oscillatory integrals.

math.FA↗

arXiv2009

Permeability of a bubble assembly: From the very dry to the wet limit

We measure the permeability of a fluidized bed of monodispersed bubbles with soap solution characteristic of mobile and non-mobile interfaces. These experimental data extend the permeability curves previously published for foam in the dry limit. In the wet limit, these data join the permeability curves of a hard sphere suspension at porosity equal to 0.4 and 0.6 in the cases of mobile and non-mobile interfaces respectively. We show that the model of permeability proposed by Kozeny and Carman and originally validated for packed beds of spheres (with porosity around 0.4) can be successfully applied with no adjustable parameters to liquid fractions from 0.001 up to 0.85 for systems made of monodisperse and deformable entities with non-mobile interfaces.

cond-mat.soft↗

arXiv2006

Faithful Polynomial Evaluation with Compensated Horner Algorithm

This paper presents two sufficient conditions to ensure a faithful evaluation of polynomial in IEEE-754 floating point arithmetic. Faithfulness means that the computed value is one of the two floating point neighbours of the exact result; it can be satisfied using a more accurate algorithm than the classic Horner scheme. One condition here provided is an apriori bound of the polynomial condition number derived from the error analysis of the compensated Horner algorithm. The second condition is both dynamic and validated to check at the running time the faithfulness of a given evaluation. Numerical experiments illustrate the behavior of these two conditions and that associated running time over-cost is really interesting.

math.NA↗