Gradient Wetting Transition from the Wenzel to Robust Cassie-Baxter States along Nanopillared Cicada Wing and Underlying Mechanism

Abstract

Gradient wettability is important for some living organisms. Herein, the dynamic responses of water droplets impacting on the surfaces of four regions along the wing vein of cicada Cryptotympana atrata fabricius are investigated. It is revealed that a gradient wetting behavior from hydrophilicity (the Wenzel state) to hydrophobicity and further to superhydrophobicity (the Cassie-Baxter state) appears from the foot to apex of the wing. Water droplets impacting on the hydrophilic region of the wing cannot rebound, whereas those impacting on the hydrophobic region can retract and completely rebound. The hydrophobic region exhibits robust water-repelling performance during the dynamic droplet impact. Moreover, a droplet sitting on the hydrophobic region can recover its spherical shape after squeezed to a water film as thin as 0.45 mm, and lossless droplet transportation can be achieved at the region. Based on the geometric parameters of the nanopillars at the hydrophilic and hydrophobic regions on the cicada wing, two wetting models are developed for elucidating the mechanism for the gradient wetting behavior. This work directs the design and fabrication of surfaces with gradient wetting behavior by mimicking the nanopillars on cicada wing surface.

Preparation and Application of Superhydrophobic Copper Mesh by Chemical Etching and In-situ Growth

“…However, there will be a large adhesive force between the droplets and the contact surface, which is not enough to explain the superhydrophobic phenomenon (such as water droplets rolling on the lotus leaf), Therefore, Cassie and Baxter ( Eq. 7 ) ( Jiang et al, 2020 ; Xie and Huang, 2020 ) assume that the water droplets on the rough surface can’t fill the grooves, thus forming a composite contact, which can better explain some phenomenon.…”

Recent Developments of Superhydrophobic Surfaces (SHS) for Underwater Drag Reduction Opportunities and Challenges

“…[169] Copyright 2018, American Institute of Physics. Lotus leaf Superhydrophobicity, self-cleaning, low adhesion [31,32] Mosquito eyes Superhydrophobicity, antifogging [33,34] Salvinia Superhydrophobicity, air-retention [35] Butterfly wings Superhydrophobicity, antireflection, directional adhesion, antifogging [36,37] Shark skin Underwater superoleophobicity, low drag, antifouling [38] Fish scales Underwater superoleophobicity [39,40] Nepenthes pitcher Superhydrophobicity [41][42][43][44][45] Gecko feet Superhydrophobicity, high adhesive, reversible adhesive [46,47] Springtails Superoleophobicity [48][49][50] Rice leaves Directional transport [51,52] Snail shell Superoleophobicity, self-cleaning [53] Rose petals Superhydrophobicity, structural color, high adhesion [54,55] Loquat leaves Anti-corrosion [56] Cicada wing Superhydrophobicity, anti-reflection [57][58][59] Water strider leg Superhydrophobicity, water-repellent, antifogging [60,61] Caterpillar Superhydrophilic; superhydrophobic [62] Nacre Underwater superoleophobicity, mechanical property, strength, [63,64] Adv. Mater.…”

Banana Leaf Surface’s Janus Wettability Transition from the Wenzel State to Cassie–Baxter State and the Underlying Mechanism

“…With the evolution of biology, a specific structure has been formed on the surface of animals and plants to adapt to the living environment, which has also become one of the hotspots of biologically efficient production management and bionics research. For example, the surface of rice leaves and butterfly wings show anisotropy [1,2]; the surfaces of prickly pear cacti (Opuntia) show age-dependent wetting properties [3]; the surface of lotus leaves show super-hydrophobicity [4], and the surfaces of cicada wings show gradient wettability along the nanopillars, which achieves an efficient fog harvest while ensuring a clean and light surface [5,6]. The "Van der Waals force" generated between the large number of fine hairs on the soles of the gecko's feet and the molecules on the surface of the object is accumulated to form a high adhesion force, which enables it to travel freely on the wall [2].…”