Self-cleaning of superhydrophobic surfaces by self-propelled jumping condensate

Abstract

The self-cleaning function of superhydrophobic surfaces is conventionally attributed to the removal of contaminating particles by impacting or rolling water droplets, which implies the action of external forces such as gravity. Here, we demonstrate a unique self-cleaning mechanism whereby the contaminated superhydrophobic surface is exposed to condensing water vapor, and the contaminants are autonomously removed by the self-propelled jumping motion of the resulting liquid condensate, which partially covers or fully encloses the contaminating particles. The jumping motion off the superhydrophobic surface is powered by the surface energy released upon coalescence of the condensed water phase around the contaminants. The jumping-condensate mechanism is shown to spontaneously clean superhydrophobic cicada wings, where the contaminating particles cannot be removed by gravity, wing vibration, or wind flow. Our findings offer insights for the development of self-cleaning materials.particle adhesion and removal | water-repellant insect wings | nanostructured interfaces | capillary forces B oth natural and synthetic superhydrophobic surfaces are believed to achieve self-cleaning by the so-called "lotus effect" (1, 2). The lotus effect typically refers to the removal of the contaminating particles by impacting and/or rolling water droplets (1, 3). The superhydrophobicity is important because of the associated large contact angle and small hysteresis (4), which promotes the rolling motion carrying away contaminants. According to the conventional wisdom of the lotus effect, the self-cleaning function will cease without incoming droplets or favorable external forces, posing severe restrictions for practical applications of superhydrophobic materials.Here, we demonstrate an autonomous mechanism to achieve self-cleaning on superhydrophobic surfaces, where the contaminants are removed by self-propelled jumping condensate powered by surface energy. When exposed to condensing water vapor, the contaminating particles are either fully enclosed or partially covered with the resulting liquid condensate. Building upon our previous publications showing self-propelled jumping upon drop coalescence (5, 6), we show particle removal by the merged condensate drop with a size comparable to or larger than that of the contaminating particle(s). Further, we report a distinct jumping mechanism upon particle aggregation, without a condensate drop of comparable size to that of the particles, where a group of particles exposed to water condensate clusters together by capillarity and self-propels away from the superhydrophobic surface.The jumping-condensate mechanism reported here offers a unique route toward self-cleaning, with potential applications ranging from microelectronic wafer cleaning to heat exchanger maintenance (7). Particle removal is often accomplished in a gas flow or a liquid stream by hydrodynamic shear stresses, which are parallel to the surface. The parallel hydrodynamic forces are not ideal in competing against the adhesive mech...

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

“…Gao et al [5] first reported that closely packed nanocones on the surface of mosquito eyes are fully nonsticky to condensed microdrops (Figure 1c-e), showing a dry-style antifogging function. Subsequently, Wisdom et al [6] reported that tiny condensate microdrops on the closely packed nanocone surface of cicada wings (Figure 1f) can selfremove by jumping via mutual coalescence, displaying a moisture self-cleaning function. Based on these inspirations, our group [50] realized the controllable fabrication of bionic polymer nanonipples and nanocones via self-ordered alumina-template-assisted nanoimprinting and chemical etching, and we demonstrated the distinct transport behaviors of condensate microdrops: coalescence but no jumping on the nanonipple surface, and coalescence-induced self-jumping on the nanocone surface.…”

“…e,f) Reproduced with permission. [6] Copyright 2013, National Academy of Sciences. g) Schematic of spore ejection in basidiomycetes.…”

“…Inspired by nature, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] the special wettability of material surfaces, which is engineered via synergistic utilization of microscale surface structures and surface chemistry has been one of the most active research fields, and it can play a key role in addressing problems related to energy, environment, resources, and health. Efforts in the past decades have explored controllable fabrication, functionality, and performance optimization of bionic superhydrophobic surfaces with a contact angle above 150°, and they have resulted in technological innovations such as self-cleaning, drag-reduction, anticorrosion, antifogging, antifreezing and oil-water separation.…”

Electrically induced drop detachment and ejection

“…1,2 Similarly, cicadas clean their superhydrophobic wings by exploiting the recoil of coalescing microdroplets, which entrain dust and other debris when they detach from the wing surface. 3 These phenomena have sparked an interest in spontaneous drop detachment and lift-off, with several authors reporting similar observations on artificial substrates. The drop recoil-and-jump mechanism has been successfully exploited for enhanced heat exchange, [4][5][6][7][8] as the spontaneous jump of coalescing droplets provides an efficient way to remove liquid from a cooler surface.…”

Advances in Bio-inspired Tribology for Engineering Applications

“…7c exhibits sufficiently high contact angles with the skin that they were easily removed from the surface by tilting the surface less than two degrees. This anti-wetting property exhibited by the gecko's skin is already being considered for many tribological applications with regard to corrosion-resistant coating, which in a better sense is immune to moisture making the surface immune to corrosion [39][40][41][42][43]. To get a better perspective of what happens when a gecko skin is impacted upon by a water droplet, a time evolution picture [38] was taken as shown in Fig.…”