TY - JOUR
T1 - Direct Nanoscopic Measurement of Laminar Slip Flow Penetration of Deformable Polymer Brush Surfaces
T2 - Synergistic Effect of Grafting Density and Solvent Quality
AU - Wang, Huan
AU - Pemberton, Jeanne E.
N1 - Funding Information:
The authors are grateful for the support of this research through a grant from the National Science Foundation (CHE-0848624). H.W. thanks the Korean Institute for Basic Science, project code IBS-R020-D1, for postdoctoral support during partial writing of this manuscript. The authors are additionally indebted to S. Granick for useful discussions.
Publisher Copyright:
Copyright © 2019 American Chemical Society.
PY - 2019/10/22
Y1 - 2019/10/22
N2 - 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.
AB - 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.
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U2 - 10.1021/acs.langmuir.9b02357
DO - 10.1021/acs.langmuir.9b02357
M3 - Article
C2 - 31558025
AN - SCOPUS:85073167279
SN - 0743-7463
VL - 35
SP - 13646
EP - 13655
JO - Langmuir
JF - Langmuir
IS - 42
ER -