Direct Nanoscopic Measurement of Laminar Slip Flow Penetration of Deformable Polymer Brush Surfaces: Synergistic Effect of Grafting Density and Solvent Quality.

Abstract

A detailed quantitative nanoscopic description of soft surfaces under dynamic flow is lacking, despite its importance. To better understand the role of surface texture in nanoscopic mass transport in complex media, we used Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM) to directly measure laminar slip flow penetration depth (slip length) on poly(N-isopropylacrylamide) (pNIPAM) thin films (50-110 nm) of different grafting densities (0.60, 0.38, and 0.27 chain/nm2) in solvents of different qualities created via cononsolvency in situ. Nontrivial synergistic interplay of grafting density and solvent quality on slip length was observed. Slip lengths are typically tens of nm (40-100 nm), increasing and then reaching a plateau with applied linear flow velocity (192-2,952 μm/s) regardless of experimental system. Slip length was systematically larger for lower density films, but the effect of grafting density was more significant in a good solvent than a poor solvent. Interestingly, however, the stagnant film thickness (polymer swollen thickness minus the slip length) collapsed to almost a singular value for a given grafting density regardless of solvent quality, likely suggesting a large gradient of segmental mobility at nonequilibrium. Moreover, we found that slip flow penetrates into soft pNIPAM surfaces more deeply in a good solvent than in a poor solvent and that this behavior was general and independent of grafting density. This behavior is counter to the notion that less interaction between a fluid (probe) and a solid surface promotes slip.