arXiv subjects

Alexandre Vilquin

Publications and source records attributed to Alexandre Vilquin.

8 recordsLinked to original sources

arXiv2026

Near-surface colloidal dynamics in jammed and slipping microgel suspensions

Jammed suspensions of soft microgel particles may exhibit slippage along smooth boundaries. Owing to their expected sub-micrometric dimensions, direct observations of dynamics within the near-surface layers supposed to be responsible for this slippage have been difficult to achieve. Here, we use total internal reflection fluorescence microscopy (TIRFM) to observe nanoparticle dynamics near glass/microgel-suspension interfaces. Indicating near-wall dynamic heterogeneity, velocity profiles for suspensions are nonlinear. These profiles tend to a constant slippage velocity at submicrometric distances from the wall, consistent with macroscopic wall slip measurements. Furthermore, nanoscale particle altitude distributions are strongly dependent on the slip velocity, revealing a dynamically-mediated and nanoscale particle-organisation effect. The collected observations give support for the existence of near-wall heterogeneity as a dominant mechanism contributing to microgel wall slip. Our work also opens new perspectives for the study of particle dynamics and organisation in complex interfacial environments.

cond-mat.soft↗

arXiv2023

Dried Blood Spot Recovery: A Microfluidic Technique for Fast Elution Without Dilution

Dried blood spot (DBS) has risen in popularity due to the ease of sampling, storing, shipping and more. Despite those advantages, recovery of the dried blood in solution for analysis is still a bottleneck as it is manual, time-consuming and leads to high dilutions. To overcome those issues, we have developed a microfluidic chip allowing reversible opening, holding hermetically DBS and forcing the elution buffer through the thickness of DBS. The new technique, validated with clinical samples, is automated, fast, robust, precise, compatible with in-line analysis and leads to a highly concentrated extraction. Moreover, by using model experiments with fluorescein solutions, we show that the elution process is governed by an advection-diffusion coupling commonly known as Taylor dispersion. This new technique could open the way to a new generation of analytical devices to quantify analytes in DBS samples for a wide range of applications.

physics.med-ph↗

arXiv2022

Nanoparticle Taylor dispersion near charged surfaces with an open boundary

The dispersive spreading of microscopic particles in shear flows is influenced both by advection and thermal motion. At the nanoscale, interactions between such particles and their confining boundaries become unavoidable. We address the roles of electrostatic repulsion and absorption on the spatial distribution and dispersion of charged nanoparticles in near-surface shear flows, observed under evanescent illumination. The electrostatic repulsion between particles and the lower charged surface is tuned by varying electrolyte concentrations. Particles leaving the field of vision can be neglected from further analysis, such that the experimental ensemble is equivalent to that of Taylor dispersion with absorption. These two ingredients modify the particle distribution, deviating strongly from the Gibbs-Boltzmann one at the nanoscale studied here. The overall effect is to restrain the accessible space available to particles, leading to a striking, ten-fold reduction in the spreading dynamics as compared to the non-interacting case.

cond-mat.soft↗

arXiv2022

Formation and dynamics of a semi vortex ring connected to a free surface

Playing a role in the locomotion of some animals such as the water strider, the formation and dynamics of a semi vortex ring connected to a free surface are experimentally investigated. This semi vortex ring is generated by the circular motion of a flat circular disk in water. Digital Particle Image Velocimetry provides velocity fields and vortex properties. We show how in a broad range of Reynolds numbers, the properties of the semi vortex rings are related to the disk characteristics. In particular, our results highlight a formation process more complex than for a complete vortex ring produced by a piston stroke. In addition to the classical rolling up at the rear of the disk, a shedding phenomenon occurs on the leading edge, producing secondary vortices. The Strouhal number related to this shedding process reveals that it comes from the free-shear instability of the boundary layer identified in the near wake of a cylinder by Bloor. Finally we show that the disk thickness affects the final properties of the semi vortex rings through emission frequency of secondary vortices.

physics.flu-dyn↗

arXiv2021

Asymptotic turbulent friction in 2D rough-walled flows

The friction f is the property of wall-bounded flows that sets the pumping cost of a pipeline, the draining capacity of a river, and other variables of practical relevance. For highly turbulent rough-walled pipe flows, f depends solely on the roughness length scale r, and the f -- r relation may be expressed by the Strickler empirical scaling f $\propto$ r$^{\frac{1}{3}}$. Here, we show experimentally that for soap film flows that are the two-dimensional (2D) equivalent of highly turbulent rough-walled pipe flows, f $\propto$ r and the f -- r relation is not the same in 2D as in 3D. Our findings are beyond the purview of the standard theory of friction but consistent with a competing theory in which f is linked to the turbulent spectrum via the spectral exponent $\alpha$: In 3D, $\alpha$ = 5/3 and the theory yields f $\propto$ r$^{\frac{1}{3}}$; in 2D, $\alpha$ = 3 and the theory yields f $\propto$ r.

physics.flu-dyn↗

arXiv2020

Near-surface rheology and hydrodynamic boundary condition of semi-dilute polymer solutions

Understanding confined flows of complex fluids requires simultaneous access to the mechanical behaviour of the liquid and the boundary condition at the interfaces. Here, we use evanescent wave microscopy to investigate near-surface flows of semi-dilute, unentangled polyacrylamide solutions. By using both neutral and anionic polymers, we show that monomer charge plays a key role in confined polymer dynamics. For solutions in contact with glass, the neutral polymers display chain-sized adsorbed layers, while a shear-rate-dependent apparent slip length is observed for anionic polymer solutions. The slip lengths measured at all concentrations collapse onto a master curve when scaled using a simple two-layer depletion model with non-Newtonian viscosity. A transition from an apparent slip boundary condition to a chain-sized adsorption layer is moreover highlighted by screening the charge with additional salt in the anionic polymer solutions. We anticipate that our study will be a starting point for more complex studies relating the polymer dynamics at interfaces to their chemical and physical composition.

cond-mat.soft↗

arXiv2020

Time dependence of advection-diffusion coupling for nanoparticle ensembles

Advection-diffusion coupling can enhance particle and solute dispersion by orders of magnitude as compared to pure diffusion, with a steady state being reached for confined flow regions such as a nanopore or blood vessel. Here, by using evanescent wave microscopy, we measure for the first time the full dynamics of Taylor dispersion, highlighting the crucial role of the initial concentration profile. We make time-dependent, nanometrically-resolved particle dispersion measurements varying nanoparticle size, velocity gradient, and viscosity in sub-micrometric near-surface flows. Such resolution permits a measure of the full dynamical approach and crossover into the steady state, revealing a family of master curves. Remarkably, our results show that the dynamics depend sensitively on the initial spatial distribution of the nanoparticles. These observations are in quantitative agreement with existing analytical models and numerical simulations performed herein. We anticipate that our study will be a first step toward observing and modelling more complex situations at the nanoscale, such as target finding and chemical reactions in nanoconfined flows, dynamical adsorption and capture problems, as well as nanoscale drug delivery systems.

cond-mat.soft↗

arXiv2014

Stokes-Einstein diffusion of colloids in nematics

We report the experimental observation of anisotropic diffusion of polystyrene particles immersed in a lyotropic liquid crystal with two different anchoring conditions. Diffusion is shown to obey the Stokes-Einstein law for particle diameters ranging from 190 nm up to 2 micron. In the case of prolate micelles, the beads diffuse four times faster along the director than in perpendicular directions. In the theory part we present a perturbative approach to the Leslie-Ericksen equations and relate the diffusion coefficients to the Miesovicz viscosity parameters.We provide explicit formulas for the cases of uniform director field and planar anchoring conditions which are then discussed in view of the data. As a general rule, we find that the inequalities ηb < ηa < ηc, satisfied by various liquid crystals of rodlike molecules, imply Dpar > Dperp.

cond-mat.soft↗