Large slippage and depletion layer at the polyelectrolyte/solid interface

Chloé Barraud (1) , Benjamin Cross (1) , Cyril Picard (1) , Fréderic Restagno (2) , Lilianne Léger (2) , Elisabeth Charlaix (1)

1 LIPhy - Laboratoire Interdisciplinaire de Physique [Saint Martin d’Hères]
2 LPS - Laboratoire de Physique des Solides

The slippage of polymer solutions on solid surfaces is often attributed to a depletion layer whose origin, thickness, and interaction with the flow are poorly understood. Using a Dynamic Surface Force Apparatus we report a structural and nanorheological study of the interface between hydrolyzed polyacrylamide solutions and platinum surfaces. Polyelectrolyte chains adsorb on the surfaces in a thin charged layer, acting as a nonattractive wall for the bulk solution. We investigate the flow of the viscoelastic solution on the adsorbed layer from the nanometer to 10 micrometers, bridging microscopic to macroscopic properties. At distances larger than 200 nanometers, the flow is well described by an apparent slip boundary condition. At smaller distance the apparent slip is found to decrease with the gap. In contrast to the apparent slip model, we show that a 2-fluids model taking into account the finite thickness of depletion layers at the non-attractive wall describes accurately the dynamic forces over 4 spatial decades of confinement. Depletion layers are found to be an equilibrium property of the interface, independent on the flow and on the confinement. Their thickness is phenomenologically described by x + 2l D with x the correlation length of the semi-dilute solutions and l D the Debye length. We interpret this result in terms of screened repulsion between the charged adsorbed layer and the bulk polyions.

Domaines: Physique [physics]; Matière Condensée [cond-mat]; Mécanique des fluides [physics.class-ph]

Chloé Barraud, Benjamin Cross, Cyril Picard, Fréderic Restagno, Lilianne Léger, et al.. Large slippage and depletion layer at the polyelectrolyte/solid interface. Soft Matter, 2019, 15 (31), pp.6308-6317. ⟨10.1039/c9sm00910h⟩. ⟨hal-04030681⟩