Superhydrophobic surfaces lotus effect

Figure 7.9 Water drop on a superhydrophobic surface showing a high apparent contact angle app- The combined effect of hydrophobicity and roughness on the right length scale, causes the Lotus effect.

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 amazing water repellent properties of many biological surfaces, especially plant leaves, have prompted great research interest. With certain leaves, when water droplets fall on the surface, they can easily pick up contaminants, roll off the surface and thereby clean the leaves. Because the first observation of this self-cleaning phenomenon occurred on lotus leaves, the effect is generally termed lotus effect . Detailed inspection of the leaves showed that micrometer and submicrometer structures as well as hydrophobic materials exist on the leaf surface. The combination of these factors leads to superhydrophobicity. Wenzel [1,2] showed that surface... 

Crick, C. R. Parkin, 1. P., A Single Step Route to Superhydrophobic Surfaces Through Aerosol Assisted Deposition of Rough Polymer Surfaces Duplicating the Lotus Effect. J. Mater. Chem. 2009,19,1074-1076. 

In addition to its influence on surface reactivity, surface structure is also seen to affect wettability on the micrometer scale, as is best illustrated by the lotus effect (see Chapter 3b). The lotus leaf is superhydrophobic, i.e. has a water contact angle of about 160°, thanks to the combination of the waxes on the surface with a characteristic dual micrometer- and nanometer-scale surface topography. Without the structure, the wax chemistry would only impart mild hydrophobicity to the surface. Superhydrophobicity comes about only when a water droplet is in contact with a rough surface with a substantial enclosure of air beneath the drop (Figure 9). This is the so-called Cassie-Baxter state, named after the authors of the work that described the contact angle of water droplets in this state by means of the equation ... 

Bhushan B, Nosonovsky M, Jung YC. (2008) Lotus effect Roughness-induced superhydrophobic surfaces. In B Bhushan (ed). Nanotrihology and Nanomechanics An Introduction, pp. 995—1072. Springer Verlag, Berlin. 

Since Barthlott and Neinhuis first reported the lotus effect, the lotus has become the archetype surface for superhydrophobicity and self-cleaning abilities (Barthlott and Neinhuis, 1997). For centuries the lotus has been known as symbol of purity in many Asian religions. For over a decade, this purity has become a phenomenon within the scientific community, with many studies having been performed with a view to determining the mechanism responsible for their self-cleaning ability. 

Bhushan, B.,Jung, C.J., Nosonovsky, M., 2010. Lotus effect surfaces with roughness-induced superhydrophobicity, self-cleaning, and low adhesion. In Springer Handbook of Nanotechnology, third ed. Springer, New York, p. 1437. 

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