Reducing the contact time using macro anisotropic superhydrophobic surfaces — effect of parallel wire spacing on the drop impact

Surfaces designed to reduce the contact time of impacting droplets are potentially of great importance for fundamental science and technological applications, for example, anti-icing, self-cleaning and heating transfer applications. Previous studies have shown that the contact time can be reduced via introducing one or several crossing macroscale wires on superhydrophobic surfaces (SHSs). However, the impacts that strike far from the wires (off-center impacts) have contact times that are equal to those obtained on SHSs. Here we demonstrate that this problem can be largely solved by using macro anisotropic SHSs (macro-aniso-SHSs)—in which the wires are parallel and macroscaled. The droplet contact time depends on the spacing between the macrostripes and is remarkably reduced by 40–50% when the spacing is comparable to the droplet size. Obvious differences in the contact time are not observed for impacts that are centered on the stripe and in the groove. The impacts centered in the groove produce new hydrodynamics that are characterized by extended spreading, easy break up and bouncing in a flying-eagle configuration. The study discusses the underlying mechanisms of the impact processes. Moreover, the effect of parallel wires on the contact time is discussed by comparing the droplet impact data for grooved rice leaves and non-grooved cabbage leaves. The enhanced drop mobility associated with the macro-aniso-SHSs should be very useful in many industrial applications. Bioinspired coatings that detach water droplets from surfaces can work twice as fast when combined with macroscale wire patterns. Recent work has shown that adding textures such as ridges or crossings to super water-repellent surfaces breaks impacting droplets into small satellites that bead up quickly, but only when initial collisions occur symmetrically. Now, using three-dimensional printing, Meirong Song, Zhichao Dong, Yilin Wang and Lei Jiang in China have produced arrays of thin parallel stripes that exert an influence over every droplet impact. Rapid testing of different arrays revealed that when the stripe separation is comparable to droplet size, the liquids spend close to 50% less time on the superhydrophobic coating. High-speed cameras also captured a new wing-shaped fragmentation and bouncing effect when wire separations were large enough to direct droplet impacts into grooves. The contact time of droplets impacting on macroscopic anisotropic superhydrophobic surfaces (macro-aniso-SHSs) decreases with an increase in the spacing and the underlying mechanism includes the mass distribution and the momentum anisotropy induced by the parralel macrostripes and macrogrooves. As the figure shows, although the impacting drop on the macro-aniso-SHS with a narrow spacing (400 μm) cannot be divided by the stripes, the anisotropy of the surface concentrates the momentum in the direction parallel to the stripes, leading to breakup and thus reducing the contact time by 15–30% compared with the contact time on the SHS and micro-aniso-SHS. For macro-aniso-SHSs with wide spacing of 1200 μm, the contact time is reduced by 40–50%. The contact time for an impact centered on the stripe is not significantly different from that in the groove, whereas the impact centered in the groove produces new hydrodynamics characterized by extended spreading, easy break-up, and flying-eagle behavior. We envision that understanding the droplet behavior on macro-aniso-SHSs not only extends our fundamental understanding of classical impacting phenomena but also has potential for a broad range of applications, such as anti-icing, self-cleaning and heating transfer.