Anisotropic wetting properties on various shape of parallel grooved microstructure

The Challenge of Superhydrophobicity: Environmentally Facilitated Cassie-Wenzel Transitions and Structural Design

Superhydrophobic materials can be used in various fields to optimize production and life due to their unique surface wetting properties. However, under certain pressure and perturbation conditions, the droplets deposited on superhydrophobic materials are prone to change from Cassie state to Wenzel state, which limits the practical applications of the materials. In recent years, a large number of works have investigated the transition behavior, transition mechanism, and influencing factors of the wetting transition that occurs when a superhydrophobic surface is under a series of external environments. Based on these works, in this paper, the phenomenon and kinetic behavior of the destruction of the Cassie state and the mechanism of the wetting transition are systematically summarized under external conditions that promote the wetting transition on the material surface, including pressure, impact, evaporation, vibration, and electric wetting. In addition, superhydrophobic surface morphology has been shown to directly affect the duration of the Cassie state. Based on the published work the effects of specific morphology on the Cassie state, including structural size, structural shape, and structural level, are summarized in this paper from theoretical analyses and experimental data.

Development of Adjustable High- to Low-Adhesive Superhydrophobicity Using Aligned Electrospun Fibers

Superhydrophobic surfaces based on electrospun fibrous structures exhibit advantages of additive manufacturing and enable the passage of gases. Compared to randomly deposited fibers, directionally aligned fibers improve the control of surface wetting by a specified fiber orientation and predictable liquid-fiber contact interface. In this article, we create superhydrophobicity with adjustable adhesion based on the understanding of droplet wetting behavior on directionally aligned fibers. Directionally aligned polystyrene fibers with different diameters and interfiber distances (l) are produced using electrospinning with a rotating fin collector. The wetting behavior of droplets on the surfaces dressed by aligned fibers is characterized, and a thermodynamic model of wetting behavior is established to guide the experimental studies. As a result, high-adhesive superhydrophobicity is achieved on weak hydrophobic substrate surfaces dressed by aligned polystyrene fibers with a diameter of 1.8 μm and l between 5 and 130 μm. Water droplets (2 μL) exhibit a maximum contact angle of 156° and adhere to the fiber-dressed surfaces by tilting upside down. Low-adhesive superhydrophobicity is achieved by introducing an additional layer of aligned fibers to increase the transition energy barrier. On the dual-layer structure with an upper-layer l of 9 μm, droplets show a contact angle of 155° and can readily roll off the surface. Moreover, increasing the upper-layer l to 15 μm reserves the surface to high-adhesive superhydrophobicity.

Cross-Species Bioinspired Anisotropic Surfaces for Active Droplet Transportation Driven by Unidirectional Microcolumn Waves

Natural evolution has endowed diverse species with distinct geometric micro/nanostructures exhibiting admirable functions. Examples include anisotropic microgrooves/microstripes on the rice leaf surface for passive liquid directional rolling, and motile microcilia widely existed in mammals' body for active matter transportation through in situ oscillation. Till now, bionic studies have been extensively performed by imitating a single specific biologic functional system. However, bionic fabrication of devices integrating multispecies architectures is rarely reported, which may sparkle more fascinating functionalities beyond natural findings. Here, a cross-species design strategy is adopted by combining the anisotropic wettability of the rice leaf surface and the directional transportation characteristics of motile cilia. High-aspect-ratio magnetically responsive microcolumn array (HAR-MRMA) is prepared for active droplet transportation. It is found that just like the motile microcilia, the unidirectional waves are formed by the real-time reconstruction of the microcolumn array under the moving magnetic field, enabling droplet (1-6 μL) to transport along the predetermined anisotropic orbit. Meanwhile, on-demand droplet horizontal transportation on the inclined plane can be realized by the rice leaf-like anisotropic surface, showcasing active nongravity-driven droplet transportation capability of the HAR-MRMA. The directional lossless transportation of droplet holds great potential in the fields of microfluidics, chemical microreaction, and intelligent droplet control system.

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

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.

Biomimetic multifunctional surfaces inspired from animals

Over millions of years, animals have evolved to a higher intelligent level for their environment. A large number of diverse surface structures on their bodies have been formed to adapt to the extremely harsh environment. Just like the structural diversity existed in plants, the same also applies true in animals. Firstly, this article provides an overview and discussion of the most common functional surface structures inspired from animals, such as drag reduction, noise reduction, anti-adhesion, anti-wear, anti-erosion, anti-fog, water capture, and optical surfaces. Then, some typical characteristics of morphologies, structures, and materials of the animal multifunctional surfaces were discussed. The adaptation of these surfaces to environmental conditions was also analyzed. It mainly focuses on the relationship between their surface functions and their surface structural characteristics. Afterwards, the multifunctional mechanisms or principles of these surfaces were discussed. Models of these structures were provided for the development of structure materials and machinery surfaces. At last, fabrication techniques and existing or potential technical applications inspired from biomimetic multifunctional surfaces in animals were also discussed. The application prospects of the biomimetic functional surfaces are very broad, such as civil field of self-cleaning textile fabrics and non-stick pots, ocean field of oil-water separation, sports field of swimming suits, space development field of lens arrays.