High-speed directional transport of condensate droplets on superhydrophobic saw-tooth surfaces

Ultrarapid fabrication of robust and versatile superhydrophobic polysiloxane coatings with superior repellency

HYPOTHESIS

Superhydrophobic surfaces have tremendous application potential in various fields. One major decisive factor that determines superhydrophobicity is building micro/nanostructures on surfaces. However, the current fabrication of micro/nanotextured superhydrophobic coatings still suffers from complex procedures and poor mechanical strength of the structures. Polysiloxanes can be effectively synthesized through a rapid acid-catalyzed hydrolysis and polycondensation from alkoxy silanes. By combining a hard polysiloxane core and a low-surface-tension silane and appropriate nanoparticles, superhydrophobic coatings with high robustness should be produced easily and quickly using a polysiloxane system.

EXPERIMENTS

Viscous methyl polyhedral oligomeric silsesquioxane (methyl-POSS) resin was obtained by reacting methytriethoxysilane with water. Superhydrophobic polysiloxane coatings were prepared by using certain amounts of methyl-POSS, dimethyldimethoxysilane (DMDMS) and SiO2 nanoparticles and a small amount of acid catalyst via different coating methods. The polysiloxane coatings were studied by atomic force and electron microscopies and by various chemical and mechanical durability tests.

FINDINGS

The coating shows superior superhydrophobicity with water contact angles θA/θR over 170° and roll-off angle α < 1°, and can be obtained within seconds to minutes under ambient conditions. Especially, it presents high mechanical robustness against sandpaper abrasion for 8.0 m on a 2000-grit sandpaper under 2.5 kPa, knife-scratching, prolonged water drop impact. The structure demonstrates high chemical stability in multiple harsh environments, including heating, strong-acid, high-salt and boiling water conditions. The coating on fabric is demonstrated to be highly effective for oil-water separation, with an efficiency over 99.7 % and showing high durability over numerous cycles.

Superhydrophobic Magnetic-Driven Reactor for Microliter Droplet Reaction Interface Visualization

The development of an efficient, convenient, and cost-effective droplet-driven reactor to observe the reaction microphenomenon is crucial for investigating the chemical reaction and synthesis mechanisms. Herein, an efficient and economical strategy by combining micro-extrusion compression molding (μ-ECM) and surface modification was proposed to fabricate a superhydrophobic magnetic-driven reactor (SMDR) for microliter droplet reaction interface visualization. The wall-like array microstructures with favorable geometric uniformity and the nano-SiO2 coating with uniform dispersion endow the SMDR with robust superhydrophobicity, featuring a contact angle of 159.5 ± 1.0° and a rolling angle of 5.1 ± 0.5°. Due to the uniform dispersion of Fe3O4 in thermoplastic elastomer (TPE), the SMDR possesses sensitive magnetic responsiveness, which can drive droplets to move rapidly, continuously, and losslessly on horizontal and inclined planes, even on a plane with an inclination angle of up to 15°. Interestingly, the SMDR was successfully used to visualize the interface formation and evolution of three simple mixing/reaction processes, which provides a convenient, efficient, and low-cost method for the study of the droplet mixing reaction process and interface visualization.

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

The coalescence-induced droplet jumping on superhydrophobic surfaces has extensive application potential in water harvesting, thermal management of electronic devices, and microfluidics. The rational design of the surface structure can influence the interaction between the droplet and the surface, thereby controlling the velocity and direction of the droplet's jumping. In this study, we fabricate the superhydrophobic surface with annular wedge-shaped micropillar arrays, examine the dynamic behavior of condensate droplets on the surface, and measure the temporal and spatial variations of droplet density, average radius, and surface coverage with wedge-shaped micropillars of varying sizes. In addition, the energy analysis of the coalescence-induced droplet jumping reveals that the two primary factors influencing the jumping are the relative size and position of the droplets and micropillars. Further numerical simulations find that the wedge-shaped micropillars cause an asymmetric distribution of pressure within the droplet and at the solid-liquid contact surface, which generates an unbalanced force driving the droplet in the gradient direction of the wedge-shaped micropillar, causing the droplet to jump off the surface with both vertical and gradient-direction velocities. The capacity of the wedge-shaped micropillar surface to transport droplets in the gradient direction increases and then decreases as the relative size of the droplets and micropillars increases. The relative position of the droplet center-of-mass line perpendicular to the bottom edge of the wedge micropillars' trapezoidal shape is more favorable for droplet transport. This work reveals the influence mechanism of surface structure on the velocity and direction of droplet jumping, and the results can guide the microstructure design of superhydrophobic surfaces, which has significant implications for the application of droplet jumping.