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We studied the wetting behavior of multiscale self-similar hierarchical wrinkled surfaces. The hierarchical surface was fabricated on poly(dimethylsiloxane) (PDMS) substrates by manipulating the sequential strain release and combined plasma/ultraviolet ozone (UVO) treatment. The generated structured surface shows an independently controlled dual-scale roughness with level-1 small-wavelength wrinkles (wavelength of 700–1500 nm and amplitude of 50–500 nm) resting on level-2 large-wavelength wrinkles (wavelength of 15–35 μm and amplitude of 3.5–5 μm), as well as accompanying orthogonal cracks. By tuning the aspect ratio of hierarchical wrinkles, the degree of wetting anisotropy in hierarchical wrinkled surfaces, defined as the contact angle difference between the parallel and perpendicular directions to the wrinkle grooves, is found to change between 3° and 9°. Through both experimental characterization (confocal fluorescence imaging) and theoretical analyses, we showed that the wetting state in the hierarchical wrinkled surface is in the Wenzel wetting state. We found that the measured apparent contact angle is larger than the theoretically predicted Wenzel contact angle, which is found to be attributed to the three-phase contact line pinning effect of both wrinkles and cracks that generates energetic barriers during the contact line motion. This is evidenced by the observed sudden drop of over 20° in the static contact angles along both perpendicular and parallel directions after slight vibration perturbation. Finally, we concluded that the observed small degree of wetting anisotropy in the hierarchical wrinkled surfaces mainly arises from the competition between orthogonal wrinkles and cracks in the contact line pinning.
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Assessment of Kerogen Wettability from Contact Angle Goniometry
Understanding the wetting properties of shale reservoirs can benefit their development for energy-related purposes and their potential for long-term carbon dioxide injection and storage. Given its potential volumetric abundance and high surface area, the wetting behavior of kerogen in shale requires assessment. Despite their known limitations, wettability studies are commonly limited to static contact angle (θ) measurements. In this Article, the conflicting factors related to the analysis and interpretation of kerogen wetting via static contact angle measurements are discussed. Contact angle data for deionized water, brine (5% NaCl), and n-dodecane are presented for seven paleomarine type-II kerogens spanning a wide range of thermal maturities (vitrinite reflectance, Ro: 0.55 to 2.75%) and chemical composition (aromatic carbon content, H/C ratio, O/C ratio). Droplets of n-dodecane instantaneously absorbed (θ* ≈ 0°) upon contact with all kerogen pellet surfaces, showing the oleophilic nature of kerogen for all maturities tested. Apparent contact angles of water with kerogen surfaces were positively correlated with H/C ratios and inversely correlated with aromatic carbon content, while the bulk and surface oxygen concentrations did not strongly correlate with the measured data. Kerogen exhibited hydrophobic (θwater > 90°) behavior, except at the highest thermal maturities. For example, the least thermally mature and most thermally mature samples studied presented apparent contact angles for water of 123 ± 15 and 59 ± 10°, respectively. Profilometry analyses showed roughness average values ranging from 0.4 ± 0.1 to 3.9 ± 0.7 μm, with the indication that sample topology can affect measured contact angles, albeit in second order as compared to sample chemistry in this study. We recommend caution when associating contact angle data alone with wetting behavior, as data obtained through sessile droplet analysis are subject to known but not always considered, caveats.
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- Award ID(s):
- 1944314
- PAR ID:
- 10508005
- Publisher / Repository:
- Energy&Fuels
- Date Published:
- Journal Name:
- Energy & Fuels
- ISSN:
- 0887-0624
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.energyfuels.3c03175
