256
Shark skin has been accepted as the main source of bioinspiration to develop and
to design the “ biomimetic skin ”. There are numerous reports on corresponding
experimental and comparative works (Bechert et al. 1985 , 1997 ; Benjanuvatra et al.
2002 ; Mollendorf et al. 2004 ; Han et al. 2008 ).
The recent report by Oeffner and Lauder ( 2012 ) contains experimental results
that suggest “that one important effect of the shark skin denticles is to enhance
thrust, and not simply to reduce drag. The overwhelming emphasis of the existing
literature on shark skin has been on drag reduction, but denticles alter vortex location. This is particularly true for the tail surface where fl ow separation and vortex
formation have been demonstrated, and could increase thrust,” (Oeffner and Lauder
2012 ; see also Wilga and Lauder 2002 , 2004 ).
Not only the fi sh skin riblets-like surface, but also the surface structure of lateral plates plays an important role for swimming. For example, “the exterior surface topography of the lateral plates of three-spine stickleback may serve a
hydrodynamic role during swimming, e.g., viscous drag reduction of turbulent
boundary layers and/or reduction of surface shear stress and skin friction,” (Song
et al. 2010 ).
5.2.1 Superoleophobicity of Fish Scale Surfaces
The next phenomenon that is also crucial for swimming behaviour in fi sh is related
to the nanolevel and surface chemistry of fi sh scale. For carps, sectorlike scales are
covered by oriented micropapillae with nanostructures, presenting not only dragreducing function but superoleophilicity in air and superoleophobicity in water
(Liu and Jiang 2011 ; Bhushan 2011 , 2012 ). “Superoleophobic surfaces—those that
display contact angles greater than 150° with organic liquids and have appreciably
lower surface tensions than that of water—are extremely rare. Calculations suggest
that creating such a surface would require a surface energy lower than that of any
known material,” (Tuteja et al. 2007 ). The superoleophobic fi sh surface originates
from the water-phase micro/nano hierarchical structures. Oil droplets on fi sh scales
in water showed antiwetting behaviour. This phenomenon was used as bioinspiration for developing of a superoleophobic and low-adhesive interface on a solid
substrate with multiscale structures, via oil/water/solid three-phase systems (see
for review Liu et al. 2012 ; Zhang et al. 2012a ). For example, as reported by Cheng
et al. ( 2012 ), an underwater pH-responsive superoleophobic surface successfully
demonstrated a reversible switch of oil-adhesion on a nanostructured poly (acrylic
acid) (PAA) surface by changing the environmental pH values. At low pH, intramolecular hydrogen bonding of PAA is formed, and results in high oil-adhesion.
As for high pH, the oil droplets can easily roll off due to the intermolecular hydrogen bonding between PAA and surrounding water. Bioinspired by the surface
structures of fi sh scales, Liu et al. ( 2009 ) have elaborated micro/nanostructure silicon surfaces exhibiting superoleophobic properties; but only if the surface was
immerged in water.
5 Materials Design Principles of Fish Scales and Armor
Shark skin has been accepted as the main source of bioinspiration to develop and
to design the “ biomimetic skin ”. There are numerous reports on corresponding
experimental and comparative works (Bechert et al. 1985 , 1997 ; Benjanuvatra et al.
2002 ; Mollendorf et al. 2004 ; Han et al. 2008 ).
The recent report by Oeffner and Lauder ( 2012 ) contains experimental results
that suggest “that one important effect of the shark skin denticles is to enhance
thrust, and not simply to reduce drag. The overwhelming emphasis of the existing
literature on shark skin has been on drag reduction, but denticles alter vortex location. This is particularly true for the tail surface where fl ow separation and vortex
formation have been demonstrated, and could increase thrust,” (Oeffner and Lauder
2012 ; see also Wilga and Lauder 2002 , 2004 ).
Not only the fi sh skin riblets-like surface, but also the surface structure of lateral plates plays an important role for swimming. For example, “the exterior surface topography of the lateral plates of three-spine stickleback may serve a
hydrodynamic role during swimming, e.g., viscous drag reduction of turbulent
boundary layers and/or reduction of surface shear stress and skin friction,” (Song
et al. 2010 ).
5.2.1 Superoleophobicity of Fish Scale Surfaces
The next phenomenon that is also crucial for swimming behaviour in fi sh is related
to the nanolevel and surface chemistry of fi sh scale. For carps, sectorlike scales are
covered by oriented micropapillae with nanostructures, presenting not only dragreducing function but superoleophilicity in air and superoleophobicity in water
(Liu and Jiang 2011 ; Bhushan 2011 , 2012 ). “Superoleophobic surfaces—those that
display contact angles greater than 150° with organic liquids and have appreciably
lower surface tensions than that of water—are extremely rare. Calculations suggest
that creating such a surface would require a surface energy lower than that of any
known material,” (Tuteja et al. 2007 ). The superoleophobic fi sh surface originates
from the water-phase micro/nano hierarchical structures. Oil droplets on fi sh scales
in water showed antiwetting behaviour. This phenomenon was used as bioinspiration for developing of a superoleophobic and low-adhesive interface on a solid
substrate with multiscale structures, via oil/water/solid three-phase systems (see
for review Liu et al. 2012 ; Zhang et al. 2012a ). For example, as reported by Cheng
et al. ( 2012 ), an underwater pH-responsive superoleophobic surface successfully
demonstrated a reversible switch of oil-adhesion on a nanostructured poly (acrylic
acid) (PAA) surface by changing the environmental pH values. At low pH, intramolecular hydrogen bonding of PAA is formed, and results in high oil-adhesion.
As for high pH, the oil droplets can easily roll off due to the intermolecular hydrogen bonding between PAA and surrounding water. Bioinspired by the surface
structures of fi sh scales, Liu et al. ( 2009 ) have elaborated micro/nanostructure silicon surfaces exhibiting superoleophobic properties; but only if the surface was
immerged in water.
5 Materials Design Principles of Fish Scales and Armor