arXiv subjects

Julien Renaudeau

Publications and source records attributed to Julien Renaudeau.

4 recordsLinked to original sources

arXiv2025

Diffusiophoretic migration of colloidal particles in sucrose gradients

Diffusiophoresis (DP) refers to the migration of particles driven by a solute concentration gradient in a liquid. Observations in the case of molecular neutral solutes are rather scarce, due to the low drift velocities in dilute solutions, and the difficulty in distinguishing DP from other phenomena in concentrated solutions. We investigated experimentally DP of dispersed colloids driven by concentration gradients of sucrose in water at relatively high concentrations, $C \simeq 1$ mol L$^{-1}$. More precisely, we designed a microfluidic chip to impose a time-dependent sucrose gradient in dead-end microchannels with minimized parasitic flows. Significant migration of the particles toward the regions of low sucrose concentration has been observed, with velocities up to a few $\mu$m s$^{-1}$. Particle tracking and Raman confocal spectroscopy were used to measure individual trajectories and the unsteady sucrose concentration profile respectively. The latter is correctly described by a diffusion equation, but with an interdiffusion coefficient that significantly depends on $C$ in the range of concentrations investigated. We then showed that a model of DP based on a steric exclusion of sucrose molecules from the particle surface with an exclusion length $R_i = 5 \pm 0.9$ angstrom (close to the characteristic size of the sucrose molecule), accounts for the observed trajectories. Possible sources for the observed scattering of our experimental data are finally discussed: Brownian motion and advection of the particles by bulk flows driven by diffusioosmosis at the channel walls and buoyancy.

cond-mat.soft↗

arXiv2025

Transient osmotic flows in a microfluidic channel: measurements of solute permeability and reflection coefficients of hydrogel membranes

We first highlight theoretically a microfluidic configuration that allows to measure two fundamental parameters describing mass transport through a membrane: the solute permeability coefficient $\mathcal{L}_D$, and the associated reflection coefficient $\sigma$. This configuration exploits the high confinement of microfluidic geometries to relate these two coefficients to the dynamics of a transient flow induced by forward osmosis through a membrane embedded in a chip. We then applied this methodology to hydrogel membranes photo-crosslinked in a microchannel with \textit{in situ} measurements of osmotically-induced flows. These experiments enable us to estimate $\mathcal{L}_D$ and $\sigma$ and their dependence on the molecular weight of the solute under consideration, ultimately leading to a precise estimate of the molecular weight cut-off of these hydrogel membranes.

cond-mat.soft↗

arXiv2025

Fluorescent molecular rotor-based polymer materials for local microviscosity mapping in microfluidic channels

A viscosity-sensitive monomer consisting of a methacrylate-functionalized julolidone-based molecular rotor (MECVJ) was synthesized and used to obtain viscosity-sensitive polymers (poly(DMA-\textit{s}-MECVJ)). The qualitative properties of the molecular rotor were preserved after its inclusion in the new polymer, in particular the effect of the viscosity of the surrounding medium on the fluorescence lifetime of the rotor. By grafting these polymers onto glass slides, viscosity-sensitive surfaces were obtained, showing good robustness in time after successive use and washing. As proof of concept, these surfaces were used to assemble a microfluidic chip capable of mapping viscosity of fluids flowing inside the channel.

cond-mat.soft↗

arXiv2022

Molecular rotors for in situ local viscosity mapping in microfluidic chips

In numerous industrial processes involving fluids, viscosity is a determinant factor for reaction rates, flows, drying, mixing, etc. Its importance is even more determinant for phenomena observed are at the micro- and nano- scales as in nanopores or in micro and nanochannels for instance. However, despite notable progresses of the techniques used in microrheology in recent years, the quantification, mapping and study of viscosity at small scales remains challenging. Fluorescent molecular rotors are molecules whose fluorescence properties are sensitive to local viscosity: they thus allow to obtain viscosity maps by using fluorescence microscopes. While they are well-known as contrast agents in bioimaging, their use for quantitative measurements remains scarce. This paper is devoted to the use of such molecules to perform quantitative, \textit{in situ} and local measurements of viscosity in heterogeneous microfluidic flows. The technique is first validated in the well-controlled situation of a microfluidic co-flow, where two streams mix through transverse diffusion. Then, a more complex situation of mixing in passive micromixers is considered and mixing efficiency is characterized and quantified. The methodology developed in this study thus opens a new path for flow characterization in confined, heterogeneous and complex systems.he methodology developed in this study thus opens a new path for flow characterization in confined, heterogeneous micro- and nano- systems.

physics.flu-dyn↗