PHD Defense - Gabriel Guyard - 13/12/2022 - Near-surface transport of polymer solutions and time-dependent soft microfluidics

Gabriel Guyard’s PhD Defense will take place on:
December 13th at 1pm
IPGG amphitheatre
6 Rue Jean Calvin, 75005 Paris

Near-surface transport of polymer solutions and time-dependent

Flows of polyacrylamide (PAM) solutions under micrometric confinement are experimentally studied. By combining microfluidics, evanescent wave microscopy and particle tracking velocimetry, we measure the velocity field within the first few hundred of nanometers in the vicinity of a glass surface.Our measurements allow to simultaneously access the rheological behavior of the sample, and the hydrodynamic boundary condition.
While the observed shear-thinning of polymer solutions is consistent with standard measurements, the boundary conditions are shown to be non-trivial and mediated by electric charges. Neutral PAM samples display a chain-sized adsorbed layer, which shifts the no-slip plane accordingly and decreases the effective size of the channel. Conversely, anionic hydrolyzed PAM solutions show apparent slip at the wall, synonymous with permeability increase. The latter was attributed to the presence of a thin low-viscosity lubrication layer close the channel surface, due to electrostatic repulsion of polymer chains.
To further characterize flows at the scale of the microfluidic device, the flow rate and pressure are measured in real time at the inlet of the elastomeric channel. Significant deformations of the conduit cause non-linear flow rate vs pressure relation, and a finite relaxation time upon pressure change, the latter attributed to a volume-storage capacity. We propose an elastohydrodynamic model to quantitatively rationalize these observations, with focus on the dynamical part of the problem. The latter is based on the lubrication equation, the local elastic response of the elastomer, and specifically takes into account the effect of the peripheral sensors.
Finally, and as a perspective, a setup able to simultaneously and dynamically achieve local microscopy and global flow measurement in complex fluids is proposed.