Diverging Effects of Topographical Continuity on the Wettability of a Rough Surface

Abstract

The effects of the continuity of the surface pattern on wetting enhancement was investigated using micropillar and microhole arrays on hydrophilic and hydrophobic materials. Isolated micropillar arrays and continuous microhole arrays were prepared by a microscale imprinting technique using positive and negative Si molds fabricated by a conventional photolithography technique. The contact angles (CAs) and contact angle hysteresis (CAH) of the prepared surfaces were measured as a function of the surface parameter ξ, defined as the ratio of the top surface area of the microstructure to the surface area of the flat unit cell. It was found that the CAs of the micropillar array monotonically increased as the surface ratio decreased, regardless of the native wettability of the solid. However, an abnormal and consistent decrease of the CAs for the microhole array was observed when ξ < 0.5. To investigate the mechanism of this abnormality in wetting enhancement, the energy barriers for normal direction wetting, the so-called wetting transition from Cassi-Baxter (CB) wetting to Wenzel wetting, and lateral direction wetting, that is, spreading, were investigated with consideration of the trapped air in the microhole. The analysis unveiled that the hydrophobicity of the hydrophilic surfaces are attributable to the liquid-air interface pinning at the discontinuous edge of the pillar, which results in CB wetting. The abnormal decrease in the CAs of the microhole-patterned surfaces with ξ < 0.5 has been attributed to the relatively low energy barrier for spreading influenced by the continuity of the three-phase contact line. Additionally, trapped air in the microhole also plays a role in the spreading of water droplets by hindering the wetting transition from CB wetting to Wenzel wetting.

Perovskite Relaxor‐Dependent Contributions to Piezoelectric Power Generation of Multiple PZT Tape‐Based Cantilever Structures at Nonresonant Frequency

“…This method differs from typical vibrational resonance techniques that seek maximum harvesting outcomes at resonant frequencies from vibrational mechanical sources in a broad range of frequency. [ 26–29 ]…”

“…For ceramic‐based piezoelectric generators, most of the reported harvesting results are based on cantilever‐type harvesters using vibrational mechanical sources over a wide range of frequencies (reaching a few hundred hertz). [ 26–28 ] The maximum harvesting performance is achieved at a specific resonant frequency (typically ranging from ≈40 to 500 Hz). This resonant frequency is determined by factors, such as the choice of piezoelectric material, device structure, vibrational source, and measurement condition.…”

Anisotropic Wettability on One-Dimensional Nanopatterned Surfaces: The Effects of Intrinsic Surface Wettability and Morphology

“…They have shown that the anisotropy can be controlled by the height of the pillars and is only observed for hydrophilic surfaces. In practice, surface patterning can be achieved by various methods such as microcontact printing, photolithography, , and vapor or cold spray surface deposition. , Other recent practical patterning techniques such as microplasma jets, electrochemical etching, and using nanometer-scale particles have shown great potential in creating surfaces with desired wetting properties. Anisotropic wettability has attracted much interest due to its importance in developing microfluidic devices, molecular separation and sensing, and self-cleaning surfaces .…”

“…Anisotropic wettability has attracted much interest due to its importance in developing microfluidic devices, molecular separation and sensing, and self-cleaning surfaces . It provides a mechanism for the directional control of liquid flow by restricting the movement of droplets in specific directions. − The existing research on anisotropic wettability has focused on macroscale water droplets. However, no studies have considered anisotropic wettability at the nanoscale for alkanes.…”

Directional Metastable Wetting Evolution of Droplets on Artificial Patterned Microcavity Surfaces

“…The entrapped air hinders the spreading dynamics and wetting transition inside the isolated microcavity. [ 35,36 ] Following the omniphobic and superomniphobic microtextures composed of reentrant and doubly reentrant pillars, [ 37–39 ] Domingues et al. [ 40,41 ] designed a landmark omniphobic surfaces with doubly reentrant cavities that can significantly improve the stability of the air–liquid interface and achieve robust entrapment of air upon immersion in wetting liquids.…”