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

Interfacial polymer flows play a significant role in many domains. To understand these complex flows, it is crucial to simultaneously characterize the mechanical behavior of the fluid as well as the boundary conditions at the interfaces. Here, near-surface flow of neutral and charged polyacrylamides were investigated using total internal reflection fluorescence microscopy (TIRFM). Near-surface polymer solutions display non-linear stress/strain-rate relation, consistent with bulk shear-thinning mechanical behavior. This near-surface rheology is accompanied with non-trivial hydrodynamic boundary conditions. Neutral polymers display a chain-sized adsorbed layer on glass that is weekly dependent of the polymer concentration and shear rate. Conversely anionic polymers show apparent slip lengths ranging from a few nanometers to several microns. The viscosity dependence of the slip length is in good agreement with a simple two-layer depletion model, which can be suppressed by the addition of salt. These results show that TIRFM is an efficient tool to study the dynamics of complex fluids at interfaces, and that effective boundary conditions can be easily tuned over several orders of magnitude in microfluidic contexts.