Title: Target slinging of droplets with a flexible cantilever
Authors: Wei Fang1,2, Shun Wang1, Hu Duan1, Shahid Ali Tahir1, Kaixuan Zhang2, Lixia Wang1, Xi-Qiao Feng2, Meirong Song1
Control of the directional bounce of droplets impacting solid surfaces is crucial for many agricultural and industrial applications. However, for the universal impact process of raindrops on plant leaves, little is known about how the highly coupled and complicated fluid–structure interaction controls the postimpact motion of droplets and endows the leaves with tenacious vitality. Here, we report a leaf-like superhydrophobic cantilever to flexibly bounce droplets with well-defined directionality and controllability. Through theoretical modeling and three-dimensional fluid–solid coupling simulations, we find that the flexible cantilever significantly relieves the impacting forces of raindrops to reduce droplet fragmentation and enhance water repellency. The results further uncover the scaling relations of the droplet bouncing direction with respect to Weber number and cantilever stiffness. By this technique, the seemed disorganized postimpact movements of droplets are programmable and predictable, achieving the goal of where to point and where to hit automatically. This work advances the understanding of natural droplet impact phenomena, opens a new avenue for delicately controlling liquid motion in space with soft materials, and inspires a plethora of applications like soft robots to transport materials and energies, monitor plant growth as well as predict pathogen transmission in plants.
Fig.1 Bioinspired directed droplet bouncing on the superhydrophobic flexible cantilever. (a) Merged snapshots from the droplet impact test on a Pogonatherum crinitum leaf (Supporting Information: Video S1). The droplet bouncing direction can be backward (a1), upward (a2), and forward (a3) on the leaf. (b) Scanning electron microscope (SEM) image of the leaf surface showing the micro and nano-mastoid structure. (c) Selected snapshots of a droplet impacting onto a superhydrophobic rigid cantilever made of glass. (d) SEM image of the glass surface showing the accumulation structure of nanoparticles. The static, advancing, and receding contact angles of water on the surface are 154.1°, 164.1°, and 150.1° (Supporting Information: Figure S1), respectively. (e) Selected snapshots of a droplet impacting onto a flexible superhydrophobic cantilever (FSHC) made of nylon. Here both the rigid and elastic cantilevers are at an inclined angle of 15°, where the impact velocity of the droplet is 1.4 m s-1. The size of the elastic cantilever is 15 mm × 5 mm × 0.13 mm, and the bending stiffness is 1.12 × 10-7 N m2. (f) SEM image of the nylon surface showing the microcross-fiber structures with nanoparticles. The static, advancing, and receding contact angles of water on the surface are 152.1°, 163.9°, and 135.5° (Supporting Information: Figure S1), respectively. (g–i) The bouncing direction of the postimpact droplet on the FSHC can be arbitrarily tuned by regulating the impact velocity V0 (g), the distance between the impact point and the fixed end P (h), and the cantilever inclined angle α (i). The bouncing trajectories and corresponding directions of droplets departing from the cantilever are separately represented by colored dots and arrows.
Fig.2 Dynamic simulation of the droplet impacting the FSHC. (a) Simulation results at the representative time showing the evolution of the droplet shape on a rigid cantilever. The arrow indicates that the droplet eventually bounces to the backside. (b) Pressure distribution in the cross-sections of the droplet at the corresponding time in (a). (c) Simulation results at the representative time showing the deformations of the droplet and the FSHC. The arrow indicates that the droplet eventually bounces to the front side. (d) Pressure distribution in the cross-sections of the droplet at the corresponding time in (c). (e, f) The force components along the x (e) and y (f) axes of the force exerted by the liquid on the rigid (in blue) and elastic cantilever (in red). The black double-headed arrow marks the difference between the local maximum (minimum) values of the two curves. The percentage is the ratio of the local maximum (minimum) value of the red curve to the blue curve. Here, each local maximum (minimum) value takes the average of the five data points. FSHC, flexible superhydrophobic cantilever.
Fig.3 Programmed high jumping and target shooting. (a) Selected snapshots to show the high jumping of droplets. The target heights from 1 to 4 are 24, 16, 8, and 0 mm, respectively. The droplet trajectory is superimposed by high-speed photography and highlighted by dotted lines, and the side view only shows the screenshot when the droplet reaches the specified height. (b) A diagram of programmed target shooting of droplets. (c) Selected snapshots to show target shotting of droplets. The coordinates of the four targets are, respectively, set as 1 (3, 7 mm), 2 (20, 6 mm), 3 (26, 10 mm), and 4 (31, 15 mm). More detailed information is given in Supporting Information: Note 3 and Video S3.
Droplet（《液滴》）是由吉林大学主办，与国际著名出版公司Wiley合作出版的英文国际性学术期刊，是国际上第一本全面报道液滴／气泡交叉领域科研成果的学术期刊。目前为季刊，主要发表液滴／气泡相关领域的原创性研究论文、综述及评论性文章，重点报道与液滴／气泡相关的结构、材料和系统设计、制备和调控等方面的基础研究及工程应用。现任主编为中国科学院院士任露泉教授、美国加利福尼亚大学洛杉矶分校C.J.Kim教授。执行主编由香港理工大学王钻开教授担任。
目前，Droplet（《液滴》）已通过全球最具影响力的开放存取期刊目录（Directory of Open Access Journals, DOAJ）评估，正式被DOAJ数据库收录。本刊旨在成为跨学科的高水平学术交流平台，展示液滴和气泡相关领域的前沿研究成果，推进国际科研传播与合作。
编辑部总编：张成春教授，副总编：王丹编审。
