Natural superhydrophobic surfaces

Figure 4. SEM images of naturally superhydrophobic surfaces (a, b) lotus leaf and (c, d) water strider leg (reprinted with permission from WUey Interscience [29] and Nature Publishing Group [31]).

Natural superhydrophobic surfaces [1, 2] as observed with leaves of some plants or feathers of some birds, correspond to hydrophobic surfaces whose water contact angle (0) is higher than 150° [3], Such surfaces are sometimes called self-cleaning surfaces since a water droplets will take up the dirt particles and roll off the surface [2], The values of water contact angles cannot be obtained with real surfaces, as... 

Y.Y. Yan, N. Gao and W. Barthlott, Mimicking natural superhydrophobic surfaces and grasping the wetting process A review on recent progress in preparing superhydrophobic surfaces, Adv. Colloid Interface ScL, 169,80-105 (2011). 

In the present chapter, we will review natural superhydrophobic surfaces, such as plant leaves, bird feathers, insect wings and legs. There after, we will present the theory of superhydrophobicity and will review biomimetic superhydrophobic surfaces. 

Li W, Amirfazli A (2008) Hierarchical structures for natural superhydrophobic surfaces. Soft Matter 4 462-466... 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

The lotus effect has inspired scientists to design superhydrophobic surfaces for applications such as self-cleaning windows and water-repellent clothing. To understand the lotus effect and other phenomena involving liquids and solids, we must understand intermoiecuiar forces, the forces that exist between molecules. Only by understanding the nature and strength of these forces can we understand how the composition and structure of a substance are related to its physical properties in the liquid or solid state. 

The fact that tuning the chemical nature alone of the soUd is unable to provide friction reduction beyond the submicrometer scale has led to the suggestion that one should try to get rid of the actual solid-liquid boundary by coating the surface with a bubble (a gas layer). Such a situation, where gas is trapped at the solid interface and partially replaces the solid-liquid contact, can be achieved in specific conditions (see Section 2.1) with the use of the so-called superhydrophobic surfaces. Such surfaces, which combine surface roughness and nonwettability to achieve unique static properties with water contact angles close to 180°, were indeed recently predicted [17] to exhibit also super-lubricating characteristics. 

In order to better quantify what affects the liquid response upon impact, Duez et al. [47] systematically measured the threshold velocity U associated with the onset of air entrainment as a function of the numerous experimental parameters sphere wettability, sphere diameter, liquid characteristics (dynamic viscosity, surface tension) or gas characteristics (nature, pressure)— We concentrate first on the role of surface wettability. Figure 4 shows the evolution off/ with the static contact angle 9q on the sphere. As already mentioned, U strongly depends on 9q, particularly in the non-wethng domain 9q > 90°) where U starts from around 7 m/s to become vanishingly small for superhydrophobic surfaces with 9q 180°. In this last case, an air cavity is always created during impact, whatever the sphere velocity. 

Another inspirational example from Mother Nature is the gecko, which can cUmb vertically even on smooth substrates [12, 13]. Even though a gecko can adhere strongly to a surface, surprisingly its tiptoe, comprising millions of setae, demonstrates superhydrophobic behavior [13]. Recently, there have been a few reports on so-called sticky superhydrophobic surfaces [14,15], obtained via aligned polystyrene hollow nanotubes [14] or etched hydrophilic aluminum alloy [15]. 

Among the various types of surface structures and roughness, the case of fibrous structure is of particular importance in many practical applications. This class of materials may have appropriate surface roughness for superhydrophobicity when the diameters of individual fibers fall in the range of micrometer (1-100 pm), submicrometer (0.1-1 pm) and nanometer (<100 nm) sizes. In the simplest cases, such materials can be envisioned geometrically as an assembly of one-dimensional cylindrical objects. In this paper, superhydrophobic surfaces composed of micro-or nanofibers found in nature as well as those made by various methods, the most prominent of which is electrospinning, are reviewed. 

Superhydrophobic surfaces (water contact angles higher than 150°) can only be achieved by a combination of hydrophobicity (low surface energy materials) with appropriate surface texture. In nature one can find an array of impressive and elegant examples of superhydrophobic surfaces. For example, on a lotus leaf rain drops bounce off after impact, then entirely roil off the lotus leaf and drag along any dirt particles, without leaving residues. 

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