The three-phase contact line shape and eccentricity effect of anisotropic wetting on hydrophobic surfaces

Abstract

This paper experimentally evaluates the combined effects of eccentricity, relative spacing, and viewing directions on the wetting conditions and the three-phase contact line shapes of hydrophobic surfaces patterned with discrete micropillars. Different techniques to depict the tortuosity of the contact line between the water droplet and microstructured surfaces are presented. First, square micropillars with different values of normalized eccentricity, , and relative spacing, , were fabricated using double casting replication technique. Subsequently, the contact angles were measured along different viewing angles by gradually rotating the sample from to . The contact angle distribution was found as a periodic function of viewing angle whose period depends on the micropillar eccentricity. The results showed that anisotropy increases by increasing the micropillar eccentricity or decreasing the pillar relative spacing. However, the effect of changing the micropillar eccentricity was much more pronounced. Micropillars with and smaller showed maximum degrees of anisotropic wetting and droplet distortion corresponding to 7% and 15%, respectively. Using the measured droplet aspect ratio, corrugated shapes of the three-phase contact line of the micropillars were also reconstructed. Finally, a simple yet effective semi-analytical model, based on Fourier series curve-fitting of the experimental data, was developed to describe the equilibrium 3D shape of the droplet on anisotropic surfaces. Experimental and simulation results reveal that the degrees of anisotropic wetting and droplet distortion were directly proportional to the energy barriers of the system resulting from noncircular corrugated shape of the three-phase contact line. The obtained results may further shed light on the underlying mechanism influencing anisotropic wetting on micropatterned surfaces.

Strain Engineering of Wave‐like Nanofibers for Dynamically Switchable Adhesive/Repulsive Surfaces

“…Static tuning of the films’ wetting properties was possible only with the parallel‐aligned wave‐like films ( Figure a,b). Owing to the impact of anisotropicity on the film wetting properties, the contact angles of the nanofibrous films were imaged from two orthogonal viewpoints, parallel and perpendicular to the fibers alignment (Figure S6, Supporting Information). The largest variations in contact angles were 6.23° (Figure S6a, Supporting Information) and occurred for the straight fibers (orthogonally alignment) films with 100% prestretch.…”

“…Wetting analysis was conducted within 48 h of synthesis. Owing to the impact of anisotropicity on the film wetting properties, the contact angles of the orthogonally and parallel‐aligned nanofibrous films were imaged (Figure S6, Supporting Information) from different orientations (0° and 90°) to the fiber alignment and assessed for variations. The CA variations (Figure S6, Supporting Information) were well within the standard batch‐to‐batch contact angle variations (1°–9°) of both orthogonally and parallel‐aligned nanofibrous films, and further contact angles during dynamic tuning were thus reported for an imaging direction of 90° to the fiber alignment only.…”

Spheroids-on-a-chip: Recent advances and design considerations in microfluidic platforms for spheroid formation and culture

“…Coating microchannel walls with a layer of poly (hydroxyethyl methacrylate) hydrogel has also been shown to optimize spheroid formation [13]. Contrastingly, as required for specific applications [94,97], PDMS also can become hydrophilic by using plasma treatment.…”

Tailoring Anisotropic Wettability Using Micro-pillar Geometry

“…Lv and Hao [19] has proposed a new type of water droplet transportation mechanism on a micro-structured hydrophobic surface and found that a water droplet could be driven by scale effect of micro-pillars under disturbance and vibration. Kashaninejad [20] has investigated the three-phase contact line shape and eccentricity effect of anisotropic wettability on pillar structures. It is found that anisotropy increases by increasing the micro-pillar eccentricity.…”