Shark skin effect

A good example to explain this impetus is the shark skin effect. The scales of predatory fishes have fine longitudinal grooves that cause less cross-flow in the displaced water. This principle has been adapted to aeroplane surfaces to help to reduce fuel consumption by 1 %. Similar shark skin swimsuit surfaces allow competitive swimmers to enjoy the advantages of lower resistance to movement through the water. 

The book b ins with a chapter that describes certain functions of the skin which contributes to the new area of bio-inspired design. The next four chapter have more specific subjects with its technological applications. Chapter 2 analyzes the shark skin effect or the commonly known antifriction surfaces. Chapter 3 discusses the Lotus effect or the usually known self-cleaning surfaces. Chapter 4 analyzes the Moth-eye effect or the commonly known antireflection surfaces and Chap. 5 describes the Gecko effect or the usually known dry adhesive surfaces. 

The shark skin effect can be avoided by using special additives such as fluoropolymers. These coat the machine and the die surfaces the wall slippage rate is increased, and thus the stick-slip effect is counteracted. 

Sharkskin shark- skin (1851) n. (1) An irregularity of the surface of an extrudate in the form of finely spaced sharp ridges perpendicular to the extrusion direction, believed to be caused by a relaxation effect on the melt at the die exit. (2) A hard-finished, twill fabric, woolen or worsted, made of... 

This partial-slip condition, over a proportionately large surface area felt by the flow passing over the sharks skin, inhibits flow reversal which would otherwise lead to separation. Thus, the overall effect for the shark would be a reduction in pressure drag. 

Common defects encountered with extrusion include effects associated with the viscoelastic nature of plastic melts. As the melt is extruded from the die for example, it may exhibit shark-skinning, melt fracture, and die swell. Diagrams of these defects are shown in Fig. 1.57. Shark-skinning and melt fracture occur when the stresses being applied to the plastic melt exceed its tensile strength. Die swell occurs due to the elastic component of the polymer melt s response to stress and is the result of the elastic rebound of the polymer as it leaves the constraints of the die channel prior to cooling. 

Bixler, G.D., Bhushan, B., 2012. Bioinspited rice leaf and butterfly wing surface structures combining shark skin and lotus effects. Soft Matter 8,11271. 

Fig. 2 shows the flow curve for the neat exact 5361 at loot). The instability problem of the metallocene based polymers with narrow molecular distribution is well known. Fig. 3 shows the photographs of the extrudate samples with varying Dechlorane concentration collected during the capillary rheometers measurement. It is interesting to see that while the severe instabilities such as slip-stick and gross-melt fracture were observed at the shear rate from 177.8 s to 3162.2 s for the neat Exact resin, the severe instability appeared at the shear rate between 177.8 s and 562.2 s but disappeared at the shear rate above 1000.1 s for Exact/10% Dechlorane suspension. The shark-skin like instabilities were observed above the Dechlorane concentration of 20% and the shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. Since the viscosity of the Dechlorane-filled systems was higher than that of the neat resin at all rates of shear, the instabilities are expected to develop the melt fracture at even lower shear rates. The shear viscosity vs. shear rate relationships measured with plate-plate rheometers and capillary rheometers are shown in Fig. 4. In this figure it is seen that both sets of data are reasonably matched. It is observed that at low shear rate range the viscosity increment due to the increase in the filler concentration is more pronounced than that at high shear rate. Both plate-plate and capillary measurements were carried out with constant shear rate (CSR) mode. While the capillary rheometer could accurately follow the preset shear rate values the plate-plate rheometer couldn t keep up with the preset shear rate values. Above two observations are due to the yield stress developed at low shear rate. At low shear rate particle-particle interaction dominates the flow phenomena and the yield stress was observed. At high shear rate hydrodynamic effect dominates the flow phenomena.

Squalene is an isoprenoid compound that is found in large quantities in shark liver and in smaller quantities in olives, rice bran and wheat germ. It is a bactericidal compound and an antioxidant and it aids in skin nutrition. Several cosmetic applications of rice bran and rice bran oil arise from the biological effects of their squalene, vitamin E and y-oryzanol content. Since these compounds are fat-soluble, rice bran oil is used for all these preparations. 

Artificial skin had been made from a bilayer fabricated from a cross-linked mixture of bovine hide, collagen, and chondroitin-B-sulfate derived from shark cartilage with a thin top layer of siloxane. The siloxane layer acts as a moisture- and oxygen-permeable support and to protect the lower layer from the outer world allowing skin formation to occur in conjunction with the lower layer. Poly(amino acid) films have also been used as an artificial skin. Research continues in search of a skin that can be effectively used to cover extensive wounds and for burn patients. 

Nowicki and Bara ska-Rybak (2007) studied the protective effect of shark liver oil. They observed a significant protection against bacterial and fungal infections by shark liver oil treatment which contains mostly squalene and alkylglycerol. Further, this treatment showed improved effects on xerosis and skin lesion-induced atopic dermatitis. This antibacterial and antifungal effect could be accounted for the high-squalene-including composition of the shark liver oil however, detailed studies are needed to be carried out for reputed activity of squalene as an anti-infectant. 

Consumption of squalene is low because of its instability, but it is used as a food supplement-folk medicine, particularly in Japan. It is claimed to have beneficial effects on the skin and in boosting the immune system though hard scientific evidence for this is somewhat lacking. When used as a food additive, it is usually in the form of shark liver... 

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