Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops

Abstract

Wind-dispersal of seeds is a remarkable strategy in nature, enlightening the construction of microfliers for environmental monitoring. However, the flight of these microfliers is greatly affected by climatic conditions, especially in rainy days, they suffer serious raindrop impact. Here, a hierarchical superhydrophobic surface is fabricated and a novel strategy is demonstrated that the superhydrophobic coating can enhance spreading while reduce contact time and impact force of raindrops, all of which are beneficial for the rotating microfliers. When the surface rotating speed exceeds a critical value, the effect of centrifugal force becomes considerable so that the droplet spreading is enhanced. The rotating superhydrophobic surface can rotate an impacting droplet by the tangential drag force from the air boundary layer, and the rotation of the droplet generates a negative pressure zone inside it, reducing the contact time by more than 30%. The impact force by the droplet on the rotating superhydrophobic surface also has a remarkable reduction of 53% compared to that on unprocessed hydrophilic surfaces, which helps maintain the flight stability of the microfliers. This work pioneers in revealing the droplet impact effect on rotating microflier surfaces and demonstrates the effectiveness of protecting microfliers with superhydrophobic coatings, which shall guide the manufacture and flight of microfliers in rainy conditions.

Understanding and Utilizing Droplet Impact on Superhydrophobic Surfaces: Phenomena, Mechanisms, Regulations, Applications, and Beyond

“…[166,167] Weakening the solidliquid contact and promoting the rapid detachment of droplets from surfaces is the primary method to achieve enhanced effect in these application scenarios. Applications such as dropletbased electricity generation, [13,168] surface cleaning, [169,170] and microfliers for environmental detections [87,171] are listed in the fourth quadrant, which all require expanding contact area, shortening contact time and boosting droplet rebound. Increasing the contact area between droplets and target objects and accelerating their separation can significantly improve the efficiency of applications.…”

“…The droplet quickly spreads outwards, and the time-varied Reproduced with permission. [87] Copyright 2022, American Chemical Society. b) Droplet spreading and retraction on the wettability-patterned SHS.…”

“…a) Isotropic spreading and retraction on the conventional SHS. Reproduced with permission [87]. Copyright 2022, American Chemical Society.…”

“…iv) The fourth quadrant (upper left) displays applications requiring short contact time, large contact area and enhanced rebound, such as surface cleaning, electricity generation (Reproduced with permission [13]. Copyright 2020, Springer Nature), and microfliers (Reproduced with permission [87]. Copyright 2022, American Chemical Society).…”

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

“…High-speed rotating surface of 𝜔 surf = 628 rad s −1 (bottom). Reproduced with permission [90]. Copyright 2022, American Chemical Society.…”