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

TL;DR

This study investigates how the charge of polymers influences their near-surface flow behavior and boundary conditions, revealing charge-dependent slip lengths and adsorption layers in semi-dilute solutions using evanescent wave microscopy.

Contribution

It demonstrates the impact of polymer charge on boundary conditions and introduces a simple two-layer depletion model to unify slip length measurements across concentrations.

Findings

Neutral polymers form chain-sized adsorbed layers on glass.

Anionic polymers exhibit shear-rate-dependent slip lengths.

Charge screening with salt transitions boundary conditions from slip to adsorption.

Abstract

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.