Coalescence-Induced Droplet Jumping on Superhydrophobic Surfaces with Annular Wedge-Shaped Micropillar Arrays

Measurement and Enhancement of Coalescence-Induced Droplet Jumping in the V-Shaped Superhydrophobic Trench with a Curved Ridge

Coalescence-induced droplet jumping demonstrates significant potential for diverse applications. However, current studies on enhancing and regulating droplet jumping largely focus on specific droplet locations and enhancement structures, significantly restricting their broader applications. This study introduces a V-shaped superhydrophobic trench with a curved ridge to enhance and control droplet coalescence jumping and directional transfer. Experimentally, a dimensionless jumping velocity (Vj* ≈ 0.69) and energy conversion efficiency (η ≈ 42.13%) were achieved, representing about 111.63% improvement in energy conversion efficiency compared to the V-shaped trench and an 879.77% increase relative to planar superhydrophobic surfaces, with a jumping angle of 66°. Numerical simulations and experiments revealed that the curved ridge enhances droplet coalescence jumping velocity and enables directional control by redirecting velocity vectors and minimizing viscous loss during coalescence. Additionally, the effects of ridge length, height, width, and opening angle on droplet coalescence jumping were analyzed via numerical simulations, offering theoretical support and technical guidance for practical applications. The coalescence jumping of droplets with unequal sizes on curved ridge structures was also studied, demonstrating that droplets with radius ratios below 0.66 can bounce off trench surfaces, confirming the structure's general applicability. This study further proposes a droplet velocity measurement method integrating target detection and trajectory fitting, achieving efficient and precise droplet velocity determination.

Coalescence-Induced Spontaneous Shedding of Microdroplets on Superhydrophobic Surfaces Featuring Enclosed Micropillars with Hierarchical Roughness

This study investigated water vapor condensation on superhydrophobic surfaces (SHSs) featuring micropillars enclosed by wall lattices and having three-tier hierarchical roughness. A total of five samples were created with three (NW-J, W200-J and W400-J samples) having large micropillar depth (∼6 μm) and two (NW-S and W200-S samples) having small micropillar depth (∼1 μm). Two distinct condensate removal modes were observed during condensation: coalescence-induced jumping on samples with large micropillar depth and coalescence-induced shedding on samples with small micropillar depth. The results showed that the diameter of the shedding droplet on the W200-S sample having small micropillar depth could be as small as 107 μm, as compared to the theoretical critical diameter of 267 μm for gravitational shedding on the same sample. The enhanced functionality of the three-tier nanotextures on the W200-S sample could effectively suppress localized pinning of the three-phase contact line and Wenzel neck formation during the growth of condensate droplets. Consequently, during multidroplet coalescence, the released surface energy easily overcomes the solid-liquid adhesion, leading to spontaneous shedding of merged droplets. The inclusion of the wall lattice aids condensate growth by the droplet self-alignment along the walls and promoting coalescence. As a result, the W200-S sample exhibited the highest condensate collection as well. The proposed surface design has great potential for scaling up and implementation in heating, ventilation, and air-conditioning equipment due to the simplicity of the surface morphology and the facile spray-coating method used to achieve hierarchical roughness.

Silica-Based Superhydrophobic and Superoleophilic Cotton Fabric with Enhanced Self-Cleaning Properties for Oil-Water Separation and Methylene Blue Degradation

Superhydrophobic textiles with multifunctional characteristics are highly desired and have attracted tremendous research attention. This research employs a simple dip-coating method to obtain a fluorine-free silica-based superhydrophobic and superoleophilic cotton fabric. Pristine cotton fabric is coated with SiO2 nanoparticles and octadecylamine. SiO2 nanoparticles are anchored on the cotton fabric to increase surface roughness, and octadecyl amine lowers the surface energy, turning the hydrophilic cotton fabric into superhydrophobic. The designed cotton fabric exhibits a water contact angle of 159° and a sliding angle of 7°. The prepared cotton fabric is characterized by attenuated total reflectance-fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. In addition, the coated fabric reveals excellent features, including mechanical and chemical stability, superhydrophobicity, superoleophilicity, and the self-cleaning ability. SiO2 nanoparticles and octadecylamine-coated cotton fabric demonstrate exceptional oil-water separation and wastewater remediation performance by degrading the methylene blue solution up to 89% under visible light. The oil-water separation ability is tested against five different oils with more than 90% separation efficiency. This strategy has the advantages of low-cost precursors, a simple and scalable coating method, enhanced superhydrophobicity and superoleophilicity, self-cleaning ability, efficient oil-water separation, and exceptional wastewater remediation performance.