Droplet impact on solid surfaces is a common phenomenon in both natural and industrial settings. The solid/liquid contact time during the collision process is one of the key factors determining the mass, momentum, and energy transfer efficiency between the solid-liquid phases. Reducing solid/liquid contact time and promoting rapid detachment of droplets from solid surfaces has always been a challenge in applications such as energy harvesting, thermal management, and anti icing. Researchers further reduce contact time by designing various gradient surfaces, which often require precise control of droplet collision points or are limited by narrow droplet collision velocity ranges.

In response to the above issues, recentlyProfessor Wang Zhuankai's team from The Hong Kong Polytechnic UniversityA dual gradient surface was designed to automatically switch to the corresponding droplet bounce mode at different collision speeds for droplets colliding with the surface. This adaptive switching of droplet bounce avoids the need for manipulation of droplet collision points and achieves rapid detachment of droplets over a larger range of droplet collision speeds. Related research content to“Self-Adaptive Droplet Bouncing on a Dual Gradient Surface”The title was published in the journal Small, with Wu Chenyang, a doctoral student at City University of Hong Kong, as the first author.

This study designed a surface with a dual gradient structure, as shown in Figure 1. Team members useNanoArch from Mofang Precision Company ® Manufacturing micro needle arrays at the micrometer level using the 1404 micro nano 3D printerThe microneedle base is 300 μ m in length, 800 μ m in length, and the microneedle spacing is 300 μ m. It exhibits good morphology and array distribution in SEM images.

The first gradient structure of the dual gradient surface is composed of continuous protruding islands with laterally asymmetric curvature, which can promote asymmetric droplet spreading and accelerate droplet rebound. The second gradient is composed of a micro conical microneedle array, with the taper spacing gradually narrowing in the vertical direction, allowing the droplet to store sufficient capillary energy during the falling process and complete rapid kinetic energy capillary energy conversion.

Through experiments, it has been shown that droplets colliding with the surface exhibit asymmetric bouncing modes at lower collision speeds (Weber number less than 9.7), reducing the solid-liquid contact time to 13.2 ms. At higher collision speeds (Weber number greater than 9.7), switching to cake bouncing mode further reduces the solid-liquid contact time to 4.5 ms. Experimental and theoretical analysis indicate that the adaptability of this bouncing mode is jointly caused by the competition between capillary pressure and dynamic water pressure, as well as the redistribution of energy during droplet spreading on the horizontal plane.

Compared with other strategies previously reported, the dual gradient surface designed in this work combines the advantages of asymmetric bounce and cake bounce, and does not require precise control of droplet collision points to achieve effective rapid droplet detachment within a larger Weber number range, providing new ideas for rapid droplet detachment.

Original link:

https://doi.org/10.1002/smll.202304635

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