Biomimetic Surfaces

Biomimetics (also referred to as bionics or biomimicry) is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology. It is estimated that the 100 largest biomimetic products generated approximately US $1.5 billion over the years 2005-2008 and the annual sales are expected to continue to increase dramatically [14]. Many biological materials have remarkable properties which can hardly be achieved by conventional engineering methods. For example, a spider can produce huge amounts (compared with the linear size of his body) of silk fiber which is stronger than steel without any access to the high temperatures and pressures which would be required to produce such materials as steel using conventional human technology. These properties of biomimetic materials are achieved due to their composite structure and hierarchical multiscale organization [30]. The hierarchical organization provides biological systems with the flexibility needed to adapt to the changing environment. As opposed to the traditional engineering approach, biological materials are grown without the final design specifications, but by using the recipes and recursive algorithms contained in their genetic code. The difference of natural versus engineering design is the difference of growth versus fabrication

[29, 54, 55]. Hierarchical organization and the ability of biological systems to grow and adapt also provides a natural mechanism for the repair or healing of minor damage in the material.

The remarkable properties of the biological materials serve as a source of inspiration for materials scientists and engineers indicating that such performance can be achieved if the paradigm of materials design is changed. While in most cases it is not possible to directly borrow solutions from living nature and to apply them in engineering, it is often possible to take biological systems as a starting point and a source of inspiration for engineering design. Molecular scale devices, superhydrophobicity, self-cleaning, drag reduction in fluid flow, energy conversion and conservation, high adhesion, reversible adhesion, aerodynamic lift, materials and fibres with high mechanical strength, biological self-assembly, antireflection, structural coloration, thermal insulation, self-healing, and sensory-aid mechanisms are some of the examples found in nature that are of commercial interest.

Biomimetic materials are also usually environmentally friendly in a natural way, since they are a natural part of the ecosystem. For this reason, the biomimetic approach in tribology is particularly promising. In the area of biomimetic surfaces, a number of ideas have been suggested [4, 14, 28, 34, 54, 68].

1. The lotus effect based non-adhesive surfaces. The term ''lotus effect'' stands for surface roughness-induced superhydrophobicity and self-cleaning. Superhydrophobicity is defined as the ability to have a large (>150°) water contact angle and, at the same time, low contact angle hysteresis. The lotus flower is famous for its ability to emerge clean from dirty water and to repel water from its leaves. This is due to a special structure of the leaf surface (multiscale roughness) combined with hydrophobic coatings, Fig. 1.2 [52, 53]. These surfaces have been fabricated in the lab with comparable performance [19, 21].

Adhesion is a general term for several types of attractive forces that act between solid surfaces, including the van der Waals force, electrostatic force, chemical bonding, and the capillary force due to the condensation of water at the surface. Adhesion is a relatively short-range force, and its effect (which is often undesirable) is significant for microsystems which have contacting surfaces. The adhesion force strongly affects friction, mechanical contact, and tribological performance of such a system's surface, leading, for example, to ''stiction'' (combination of adhesion and static friction [8, 16]), which precludes microelectromechanical switches and actuators from proper functioning. It is therefore desirable to produce non-adhesive surfaces, and applying surface microstructure mimicking the lotus effect has been successfully used for the design of non-adhesive surfaces, which are important for many tribological applications. In some applications, high adhesion surfaces are of interest. High adhesion surfaces have been produced using the so-called ''Petal effect,'' Fig. 1.3 [18, 20].

Fig. 1.2 a SEM micrographs (shown at three magnifications) of lotus (Nelumbo nucífera) leaf surface, which consists of microstructure formed by papillose epidermal cells covered with epicuticular wax tubules on the surface, which create nanostructure; b image of water droplet sitting on the lotus leaf r211

Fig. 1.2 a SEM micrographs (shown at three magnifications) of lotus (Nelumbo nucífera) leaf surface, which consists of microstructure formed by papillose epidermal cells covered with epicuticular wax tubules on the surface, which create nanostructure; b image of water droplet sitting on the lotus leaf r211

Fig. 1.3 Optical micrographs of water droplets on Rosa, cv. Bairage at 0 and 180° tilt angles. Droplet is still suspended when the petal is turned upside down [18]

Fig. 1.3 Optical micrographs of water droplets on Rosa, cv. Bairage at 0 and 180° tilt angles. Droplet is still suspended when the petal is turned upside down [18]

2. The Gecko effect, which stands for the ability of specially structured hierarchical surfaces to exhibit controlled adhesion. Geckos are known for their ability to climb vertical walls due to a strong adhesion between their toes and a number of various surfaces. They can also detach easily from a surface when needed (Fig. 1.4). This is due to a complex hierarchical structure of gecko feet surface. The Gecko effect is used for applications when strong adhesion is needed (e.g., adhesive tapes) or for reversible adhesion (e.g., climbing robot) [13, 54].

Fig. 1.4 Tokay gecko has the ability to climb walls and detach from surfaces easily at will

3. Microstructured surfaces for underwater applications, including easy flow due to boundary slip, the suppression of turbulence (the shark-skin effect, Fig. 1.5), and anti-biofouling (the fish-scale effect). Biofouling and biofilming are the undesirable accumulation of microorganisms, plants, and algae on structures which are immersed in water. Conventional antifouling coatings for ship hulls are often toxic and environmentally hazardous. On the other hand, in living nature there are ecological coatings (e.g., fish scale), so a biomimetic approach is sought [25, 27, 32, 40, 56].

4. Oleophobic surfaces capable of repelling organic liquids. The principle can be similar to superhydrophobicity, but it is much more difficult to produce an oleophobic surface, because surface energies of organic liquids are low, and they tend to wet most surfaces [48, 56, 72, 73]. Underwater oleophobicity can be used also to design self-cleaning and antifouling surfaces, Fig. 1.6 [39].

5. Microstructured surfaces for various optical applications, including non-reflective (the Moth-eye effect), highly reflective, colored (in some cases, including the ability to dynamically control coloration), and transparent surfaces. Optical surfaces are sensitive to contamination, so the self-cleaning ability should often be combined with optical properties [14, 33, 54].

6. Microtextured surfaces for de-icing and anti-icing (Fig. 1.7). De-icing (the removal of frozen contaminant from a surface) and anti-icing (protecting against the formation of frozen contaminant) are significant problems for many applications that have to operate below the water freezing temperature: air-crafts, machinery, road and runway pavements, traffic signs and traffic lights, etc. The traditional approaches to de-icing include mechanical methods, heating, the deposition of dry or liquid chemicals that lower the freezing point of water. Anti-icing is accomplished by applying a protective layer of a viscous anti-ice fluid. All anti-ice fluids offer only limited protection, dependent upon frozen contaminant type and precipitation rate, and it fails when it can no longer absorb the contaminant. In addition to limited efficiency, these de-icing fluids, such as propylene glycol or ethylene glycol, can be toxic and raise

Fig. 1.5 Scale structure on a Galapagos shark (Carcharhinus galapagensis; [64])

Fig. 1.5 Scale structure on a Galapagos shark (Carcharhinus galapagensis; [64])

environmental concerns. Anti-icing on roadways is used to prevent ice and snow from adhering to the pavement, allowing easier removal by mechanical methods.

Ice formation occurs due to the condensation of vapor phase water and further freezing of liquid water. For example, droplets of supercooled water that exist in stratiform and cumulus clouds crystallize into ice when they are struck by the wings of passing airplanes. Ice formation on other surfaces, such as pavements or traffic signs also occurs via the liquid phase. It is therefore suggested that a water repellent surface can also have de-icing properties [24]. When a super-hydrophobic surface is wetted by water, an air layer or air pockets are usually kept between the solid and the water droplets. After freezing, ice will not adhere to solid due to the presence of air pockets and will be easily washed or blown away.

7. Microelectromechanical system (MEMS)-based dynamically tunable surfaces for the control of liquid/matter flow and/or coloration (for example, mimicking the coloration control in cephalopods), used for displays and other applications, the so-called ''origami'' [23, 70].

8. Various biomimetic microtextured surfaces to control friction, wear and lubrication [14, 15, 74].

9. Self-lubricating surfaces, using various principles, including the ability for friction-induced self-organization [57].

Fig. 1.6 a Schematics of a solid-water-oil interface system. A specimen is first immersed in water, and then an oil droplet is gently deposited using a microsyringe, and the static contact angle is measured; b opticalmicrographs of droplets at three-different-phase interfaces on a micropatterned surface (shark skin replica) without and with C20F42 [39]

Fig. 1.6 a Schematics of a solid-water-oil interface system. A specimen is first immersed in water, and then an oil droplet is gently deposited using a microsyringe, and the static contact angle is measured; b opticalmicrographs of droplets at three-different-phase interfaces on a micropatterned surface (shark skin replica) without and with C20F42 [39]

Fig. 1.7 The principle of applying of surface microstructure for de-icing

10. Self-repairing surfaces and materials, which are able to heal minor damage (cracks, voids), [57, 58].

11. Various surfaces with alternate (and dynamically controlled) wetting properties for micro/nanofluidic applications, including the Darkling beetle effect, e.g., the ability of a desert beetle to collect water on its back using the hydrophilic spots on the otherwise hydrophobic surface of its back [54, 61, 63], Fig. 1.8.

12. Water strider effect mimicking the ability of insects to walk on water using the capillary forces. The hierarchical organization of the water strider leg surface plays a role in its ability to remain dry on water surface, Fig. 1.9 [31].

Fig. 1.8 The water-capturing surface of the fused overwings (elytra) of the desert beetle Stenocara sp. a Adult female, dorsal view; peaks and valleys are evident on the surface of the elytra; b SEM image of the textured surface of the depressed areas [61]

Fig. 1.8 The water-capturing surface of the fused overwings (elytra) of the desert beetle Stenocara sp. a Adult female, dorsal view; peaks and valleys are evident on the surface of the elytra; b SEM image of the textured surface of the depressed areas [61]

13. The ''sand fish'' lizard effect, able to dive and "swim" in loose sand due to special electromechanical properties of its scale [54, 63].

14. Composite and nanocomposite materials tailored in such way that they can produce required surface properties, such as self-cleaning, self-lubrication, and self-healing. Metal-matrix composites, and polymeric composites as well as ceramics (including concrete) have been recently used for this purpose. Natural fiber-reinforced composites are among these materials. The difference between microstructured surfaces and composite materials is that the latter have hydrophobic reinforcement in the bulk and thus can be much more wear-resistant than microstructured surfaces, which are vulnerable even to moderate wear rates.

15. Green biomimetic nanotribology, including cell adhesion, nanoornamentics, and biochemistry is another new area associated with green tribology.

Fig. 1.9 a Water strider (Pond skater, G. remigis) walking on water; b SEM images of a pond skater leg showing (top) numerous oriented microscale setae and (bottom) nanoscale grooved structures on a seta [31]

Water strider walking on the water

SEM images of a water strider ieg (Gao and Jiang, 2004) (b)

Environmental engineers have only just started paying attention to biomimetic surfaces. Raibeck et al. [62] investigated the potential environmental benefits and burdens associated with using the lotus effect based self-cleaning surfaces. They found that while the use phase benefits are apparent, production burdens can outweigh them when compared with other cleaning methods, so a more thoughtful and deliberate use of bio-inspiration in sustainable engineering is needed. Clearly, more studies are likely to emerge in the near future.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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