The Lotus effect

3 Wetting in the Cassie-Baxter State 4.3.1 The Lotus Effect 

Although the chemical composition of plant wax has not been well characterized, Cheng and co-workers [40] found that the wax material on the Lotus is intrinsically, moderately hydrophilic with a water contact angle of 74°. As will be discussed in the next two sections, re-entrant structure and multi-scale roughness at 

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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 simulates the properties of the lotus flower in nature, by microstructured hydrophobic protrusions which enable surfaces to clean themselves by water in motion. By making use of innovative technologies, the Lotus Effect can be used on many different products. For example, self-cleaning surfaces could be used for self-adhesive films, injection moulded parts or painted components in the construction industry, for facade elements or window frames, for traffic facilities such as road signs, and not to forget cars themselves. 

Plasma Treatment of Textile Fibers Treatment of Cotton and Synthetic Textiles and the Lotus Effect... 

Plasma-induced hydrophobization of cottonfabric in conjunction with increased specific surface area leads to an interesting and practically important effect. Water droplets are able to effectively remove dirt particles from the surface of the cotton fabric. This phenomenon is illustrated in Fig. 9-29 for the case of HMDSO-plasma-treated cotton fabric (Hocker, 2002) and is usually referred to as the Lotus effect. Thus, the highly hydrophobic plasma-treated surface of cotton with specific plasma-modified surface topography is extremely dust- and dirt-repellant in contact with water. As an important consequence, the plasma-treated surface also becomes repellant to bacteria and fungi. The effect is relevant not only to cotton fiber but to some other materials as well. 

Gu ZZ, Uetsuka H, Takahashi K, Nakajima R, Onishi H, Fujishima A, Sato O (2003) Structiu-al color and the lotus effect. Angew Chem Int Ed 42 894... 

The opposite of super-hydrophobicity, the Lotus Effect, is super-oleophobicity, the Pitcher Plant Effect. A natural example of super-oleophobicity involves the Nepenthes Pitcher Plant, which has microtextured surfaces in which an aqueous liquid fills the spaces within the texture and forms a continuous overlying film to cause insects to slip into the plant s digestive juices. Here, the plant s super-oleophobic surface essentially repels the oils on the insects feet. This is termed the Pitcher Plant Effect. Microporous, microfibre coatings have been developed to mimic this effect and be highly repellent to oils while remaining permeable to water. 

Accordingly, this self"cleanii effect is termed the Lotus Effect. 

In this chapter, we will discuss how the chemical and physical properties of substances at interfaces differ from those in the bulk. For quantitative description, quantities like surface tension and surface energy have to be introduced. With the help of these quantities, phenomena known from everyday life like the lotus effect can be explained. However, perhaps you are more interested to learn how detergents clean Then have a look at Sect. 16.3 which deals with the adsorption on liquid surfaces. The next section covers the adsorption on solid surfaces and the variation of the extent of coverage with pressure or concentration of the substance to be adsorbed. Langmuir s isotherm, the simplest description of such an adsorptiOTi process, is deduced by kinetic interpretation of the adsorption equilibrium. Alternatively, it can be derived by introducing the chemical potential of free and occupied sites and cmisideiing the equilibrium condition. In the last part of the chapter, some important applications such as surface measurement and adsorption chromatography are discussed. 

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. 

Patankar, N.A., 2004. Mimicking the lotus effect influence of double roughness structures and slender pillars. Langmuir 20 (19), 8209—8213. 

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 ... 

Beyond the Lotus Effect Roughness Influences on Wetting over a Wide Surface-Energy Range... 

Marmur A. The Lotus effect superhydrophobicity and metastability. Langmuir 2004 20(9) 3517-9. 

It has to be noted that all technical products which claim to show the Lotus effect suffer from certain drawbacks. One of the most important is the fact that these surfaces do not show the same behavior against oily liquids which destroys the effect significantly. Moreover, incrustation by algae cannot be suppressed, again reflecting the importance of this topic. Thus merging these two scientific topics and enhancing... 

Spori, D. M., Drobek, T., Zurcher, S., Ochsner, M., Sprecher, C., Muhlebach, A., and Spencer, N. D. 2008. Beyond the Lotus Effect Roughness Influences on Wetting over a Wide Surface-Energy Range. Langmuir 24 5411. 

MD simulations of the wetting of hydrophobic substrates with surface inhomogeneities displayed a similar behavior as hydrophobic rough surfaces, with high contact angles (>130° for fluid comprised of "small-chain" molecules). Upon application of a lateral push, the droplet detached from the surface, akin to the lotus effect but at the nanoscale. This was not observed with the simulations with a homogenous surface. 

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