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Near-surface rheology and hydrodynamic boundary condition of semi-dilute polymer solutions
Gabriel Guyard Alexandre Vilquin Nicolas Sanson Frederic Restagno Joshua D. Mcgraw
Soft Matter - 17 3765-3774 - doi: 10.1039/D0SM02116D - 2021
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.
Time dependence of advection-diffusion coupling for nanoparticle ensembles
Alexandre Vilquin Vincent Bertin Pierre Soulard Gabriel Guyard Elie Raphaël Frederic Restagno Thomas Salez Joshua Mcgraw
Phys. Rev. - 6 064201 - doi: 10.1103/PhysRevFluids.6.064201 - 2021
Particle transport in fluids at micro-and nano-scales is important in many domains. As compared to the quiescent case, the time evolution of particle dispersion is enhanced by coupling: i) advection along the flow; and ii) diffusion along the associated velocity gradients. While there is a well-known, long-time limit for this advection-diffusion enhancement, understanding the short-time limit and corresponding crossover between these two asymptotic limits is less mature. We use evanescent-wave video microscopy for its spatio-temporal resolution. Specifically, we observe a near-surface zone of where the velocity gradients, and thus dispersion, are the largest within a simple microfluidic channel. Supported by a theoretical model and simulations based on overdamped Langevin dynamics, our experiments reveal the crossover of this so-called Taylor dispersion from short to long time scales. Studying a range of particle size, viscosity and applied pressure, we show that the initial spatial distribution of particles can strongly modify observed master curves for short-time dispersion and its crossover into the long-time regime.
Self-Similar Relaxation of Confined Microfluidic Droplets
Margaux Kerdraon1, Joshua D. McGraw1, Benjamin Dollet2, and Marie-Caroline Jullien
Phys. Rev. Lett. - 123 24501 - doi.org/10.1103/PhysRevLett.123.024501 - 2020
We report an experimental study concerning the capillary relaxation of a confined liquid droplet in a microscopic channel with a rectangular cross section. The confinement leads to a droplet that is extended along the direction normal to the cross section. These droplets, found in numerous microfluidic applications, are pinched into a peanutlike shape thanks to a localized, reversible deformation of the channel. Once the channel deformation is released, the droplet relaxes back to a pluglike shape. During this relaxation, the liquid contained in the central pocket drains towards the extremities of the droplet. Modeling such viscocapillary droplet relaxation requires considering the problem as 3D due to confinement. This 3D consideration yields a scaling model incorporating dominant dissipation within the droplet menisci. As such, the self-similar droplet dynamics is fully captured.

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3 publications.