Self-removal of condensed water on the legs of water striders

Abstract

The ability to control drops and their movements on phobic surfaces is important in printing or patterning, microfluidic devices, and water-repellent materials. These materials are always micro-/nanotextured, and a natural limitation of repellency occurs when drops are small enough (as in a dew) to get trapped in the texture. This leads to sticky Wenzel states and destroys the superhydrophobicity of the material. Here, we show that droplets of volume ranging from femtoliter (fL) to microliter (μL) can be self-removed from the legs of water striders. These legs consist of arrays of inclined tapered setae decorated by quasi-helical nanogrooves. The different characteristics of this unique texture are successively exploited as water condenses, starting from selfpenetration and sweeping effect along individual cones, to elastic expulsion between flexible setae, followed by removal at the anisotropic leg surface. We envision that this antifogging effect at a very small scale could inspire the design of novel applicable robust water-repellent materials for many practical applications.water strider | water repellency | self-propulsion | antifogging | breath figure W hen a vapor condenses on a rough hydrophobic surface, liquid nuclei naturally appear within the roughness, forming tiny droplets stuck in the texture (1-3). Without the assistance of external forces, dew tightly adheres to the surface in the so-called Wenzel state, which most often destroys the superhydrophobicity of the material (4, 5). An applicable robust water-repellent surface thus requires an additional mechanism to expel these condensates from the texture (6). It has been reported in the literature that nanotextures (such as found on the surface of lotus leaves or cicada wings) might generate adhesion forces small enough to permit an efficient conversion of surface energy (coming from droplet coalescence) into kinetic energy, so that droplets can be mobilized (7-9). However, contact angle hysteresis effects are generally dominant at a small scale, so that a key challenge for antifogging materials lies in the generation of a force able to expel droplets from microtextures.Water striders (Gerris remigis, Fig. 1A) living at the water surface in a highly humid environment offer a remarkably simple solution to this problem. Without any external force, tiny condensed droplets in the range of femtoliters (fL) to microliters (μL) get removed from striders' legs, owing to the presence of oriented conical setae. A Gerris leg is a centimeter-size cylinder (of typical diameter 150 μm) decorated by an array of inclined tapered hairs characterized in Fig. 1 B and C by micro X-ray computed tomography (XCT) and scanning electron microscopy (SEM). Individual setae have a length L = 40-50 μm, a maximum diameter of ∼3 μm, and an apex angle of ∼5°. They make regular arrays with a mutual distance of 5-10 μm, and are tilted by an angle β = 25-35°to the base of the leg (Fig.

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

“…In essence, the ejection of the spore is driven by the coalescence of the condensed droplet, and the released surface energy can be effectively transferred to kinetic energy. Note that the CMDSP phenomenon has also been found in other organisms, such as the legs of the water strider [4] and butterfly wings. [9] 3.…”

“…[25,85,[110][111][112][113][114] The latest nanobionic research shows that the inclined setae on the legs of the water strider [4] (Figure 9c,d) and ratchet-like microstructures on butterfly wings [9] (Figure 9e,f) have the remarkable ability to directionally transport condensed microdrops. The next issue is to develop metal-based CMDSP surfaces with rationally designed wettability-contrast patterns and asymmetrical structures at the microscale, in order to achieve more efficient condensation heat-transfer performance.…”

“…c,d) Reproduced with permission. [4] Copyright 2015, National Academy of Sciences. e) Photograph of a butterfly.…”

“…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.…”

Bioinspired Dynamic Wetting on Multiple Fibers

“…[2] The cactus spine was recently reported to be capable of collecting and directionally transporting droplets, [3] and spider silk can efficiently harvest water from humid air. [4] The distinctive conical structure of the individual natural fiber www.advmat.de www.advancedsciencenews.com on water strider legs, [12] directional liquid transport inspired by the shorebird beak, [13] controllable liquid transfer inspired by the Chinese brush, [14] highly efficient liquid encapsulation inspired by dandelion pappi, [15] and capillary-induced fiber self-assembly inspired by wet hairs. [16] We focus on multiple fibrous systems with different spatial distributions, which are more complicated and challenging but offer more possibilities for practical applications.…”