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Direct Nanoscopic Measurement of Laminar Slip Flow Penetration of Deformable Polymer Brush Surfaces: Synergistic Effect of Grafting Density and Solvent Quality
journal contribution
posted on 2019-10-10, 14:05 authored by Huan Wang, Jeanne E. PembertonA 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.
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flow penetration depthSolvent Qualityreflectance fluorescence microscopynanoscopic mass transportGrafting DensitypNIPAM surfacesDirect Nanoscopic MeasurementFRET-TIRFMfilm thicknessmeasure laminarsurface texturedensity filmsSynergistic EffectSlip lengthsnmnanoscopic descriptionLaminar Slip Flow PenetrationSlip lengthDeformable Polymer Brush Surfaces
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