Small degree of anisotropic wetting on self-similar hierarchical wrinkled surfaces

Abstract

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.

Emergent Soft Lithographic Tools for the Fabrication of Functional Polymeric Microstructures

“…These methods overcome critical challenges related to the production of 3D PMs and highlight the important role that dynamic, selfforming masters will play in expanding the reach of soft lithography to different fields and technologies. These techniques support a diverse range of applications, including, but not limited to: microelectronics, [91,130,144] superhydrophobic surfaces, [73,89,90,[104][105][106] micro-lenses, [93,95] open channel microfluidics, [92] structural color, [104,105,115,124,125] cellular growth environments, [137,145,146,153] and triboelectronics. [82][83][84]128] We anticipate that this list will continue to grow as more scientists adopt the strategies represented by these new fabrication techniques.…”

“…[88] Surfaces with hierarchical wrinkles can be used as masters for replica molding in order to fabricate PMs with hierarchically wrinkled surfaces ( Figure 2D). Due to the presence of multiple wavelengths of wrinkles, these PMs can be used as dynamic superhydrophobic surfaces [73,89,90] that retain the ability to shed water when twisted or bent. [72] In addition to applications in superhydrophobic surfaces, PMs based on hierarchal wrinkles have applications in the creation of microcircuits [91] and the fabrication of open channel microfluidic devices.…”

“…AFM micrograph and corresponding line scans (indicated by dotted lines) indicate the presence of 2 wavelengths of wrinkles. Adapted with permission from Ref [90]…”

Free‐Standing Buckle‐Delaminated 2D Organic Nanosheets with Enhanced Mechanical Properties and Multifunctionality

“…The top row of Figure a shows the isotropic and hydrophilic wetting behavior of a 5 µL droplet on the planar spin‐coated and free‐standing films before releasing the prestrain, where the static contact angle of the spin‐coated film (80°) is slightly higher than that of the free‐standing sheet (71°). After the prestrain release, both buckled films show an anisotropic and hydrophobic wetting behavior due to their highly anisotropic and high‐aspect‐ratio grooved surface features . We observe that the water droplets elongate along the grooves of the wrinkles or blisters.…”

“…The buckle‐delaminated free‐standing sheet exhibits a higher degree of wetting anisotropy with Δθ = 23°, which is over twice larger than the wrinkled surface (Δθ = 11°). It should be noted that both wrinkles and cracks contribute to the wetting behavior of the wrinkled film since the transverse cracks expose the underlying hydrophobic PDMS substrate, whereas the wetting behavior of the crack‐free buckle‐delaminated sheet is mainly determined by its buckled surface features. Thus, compared to the wrinkled film, the large enhancement in both degree of wetting anisotropy and parallel contact angle in the buckle‐delaminated sheet is mainly attributed to its higher aspect ratio of the blisters, which is consistent with previous studies on the enhanced effect of high‐aspect‐ratio surface features on the wetting anisotropy …”