  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.
