The dream of staying clean: Lotus and biomimetic surfaces

Abstract

The Lotus has been the symbol of purity for thousands of years; contaminations and pathogens are washed off the surfaces of Lotus and some other plants with rain or even dew. After the introduction of scanning electron microscopy, we were able to resolve the mechanism behind this phenomenon. It took some further decades before in-depth studies on self-cleaning with plants were conducted and the effect could be understood in detail. We identified extreme water-repellency ('superhydrophobicity'), characterized by very high contact angles and low sliding angles, as the prerequisite for self-cleaning properties. We could show that the combination of two factors is necessary for obtaining a high degree of water-repellency: (1) low energy surfaces being hydrophobic and (2) surface structures that significantly increase hydrophobicity. It is suggested that this mechanism plays an important role in the protection of plants against pathogens. Our technological application of this effect has resulted in the development of successful, eco-friendly and sustainable industrial products. Another interesting property was found with superhydrophobic surfaces of certain aquatic and semi-aquatic plants and animals: here a layer of air under water is retained. We present a new approach of using this feature for creating structured, air-retaining surfaces for technical underwater applications. It is proposed that such surfaces can reduce significantly the drag of large ships. We conclude that basic biological research is of particular importance for true innovation. Our research on superhydrophobic self-cleaning biological surfaces and the development of similar engineered materials suggests that biomimicry is a matter of multi-stage processes rather than a simple copying of biological developments.

Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity

“…This explains the essential function of the heterogeneous structure of the Salvinia: the hydrophobic interior stabilizes the Cassie state with respect to liquid intrusion (Cassie-Wenzel transition), while the hydrophilic top hinders gas nucleation (details in the following). This is our main result, which at the same time clarifi es in quantitative terms the function of a complex biological structure, fi rst described by Barthlott and co-workers, [ 2,6 ] and suggests how to exploit it in the design of simpler bioinspired surfaces. Figure 2 a addresses the effect of the pressure on the free energy profi les, which amounts to adding to Ω(Φ) a term ∼ΦΔ P ; [ 21,28 ] this linear shift changes the location of the minima and determines the stability of the Cassie state: for instance, increasing the pressure always favors the Wenzel state.…”

“…The maximum ( t ransition s tate) separating two minima defi nes two free energy barriers, a "forward" and a "backward" one: Inspired by nature, [1][2][3] the study of superhydrophobicity has fl ourished in the last two decades allowing for an improved control of the wetting properties of surfaces of technological interest. [ 4,5 ] In particular, submerged superhydrophobicity is emerging as a means to reduce drag and prevent biofouling: such applications require robust gas-trapping inside surface asperities.…”

“…Summing up, the stability of a given state and the kinetics of the transition to another state are ruled by the free energy landscape through Equations ( 1) and ( 2) .…”

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

“…79 The now commonly accepted meaning of a superhydrophobic surface is a surface on which the water (advancing) contact angle is at least 150°, and the contact angle hysteresis as well as the sliding (or rolling off) angle (sliding/rolling angle is the minimum angle of sloped solid at which water (liquid) drop rolls off the surface) do not exceed 5-10° (Table 2). Although currently superhydrophobic surfaces are inspired by biological specimens, [80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97] the early research was inspired by the practical need to enhance coating repellency of water and snow. 98,99 These days, superhydrophobic coatings are manufactured by chemical, physical and/or mechanical modifications of both organic and inorganic materials.…”

Mimicking biopolymers on a molecular scale: nano(bio)technology based on engineered proteins

“…In addition to ice nucleation, this coating also displayed improved ice-repellent properties compared with commercial coatings. Another biomimetic approach to anti-icing surfaces has been performed with superhydrophobic coatings inspired by the sacred lotus leaf (Solga et al 2007). The biomimetic surfaces based on commercially available lotus-effect paints were good at self-cleaning, but showed poor anti-icing properties (V. Stenzel 2007, unpublished results).…”