Research On Material Characteristic And Tribological Behavior For Ventral Scale Of Snakes

With the development of science and technology,the bionics has been widely used in engineering and the research on biomaterial micrograph, mechanical properties and tribological behavior received great attention from micro area to macro one in the past few decades.The snakes' ventral scale has the micro non-lubricity and hydrophobe properties.It plays a great role in the snake body movement and keeps the body away from the clay,maintains the body clean.All these biological properties give a hand to figure out the problem of how to decrease the conglutination,less the resistance and overcome friction anisotropies.This paper focuses on the ventral scale research on Burmese python,Zaocys dhumnades and Elaphe rufodoreata.The contact model of the ventral scale friction movement is founded on the work of material component analysis,microstructure, surface characteristic and micro & macro tribological behavior.It aim is to understand the ventral scale material characteristic and tribological behavior and to reveal the ventral scale friction mechanism so that the bionics fabrication on ventral scale can be made.Firstly,the ingredients of ventral scale are analyzed by Fourier transform Infrared spectrometer.The results indicate that spectrums of the three ventral scale are extremely similar and the main structure isα-helix keratin.Texture analyzer is used to test the tension mechanical character of the ventral scale material.It shows the elastic modulus of the ventral scale is about 1.5 GPa and the fission strength is lower than 90 MN/m~2.The material of ventral scale has moisture absorption ability,the rate of which is about 8%.The elastic modulus of the python's ventral scale is the highest of all while that of the Zaocys dhumnades is the lowest.The elastic modulus of the damp sample is 1/5 to 1/3 of the dry one.All conclusions above all aim to know the material characteristic and to provide the basic resource of bio-fabrication.The topography of ventral scale surface is obtained with SEM and AFM.Regularly arrayed micro-structure is on the snake ventral scale surface.The structure is observed as a structure comprised of periodically arrayed micro-convex,micro-pits,micro-holes and plate-like part.Eleven parameters are used to depict the topography quantificationally.The micro-topography of ventral scale surface varies as species.The micro-convex on ventral scale surface of python is finger-like convex,and its height is 200 nm.The one of the Elaphe rufodoreata and Zaocys dhumnades is triangle and its height is 60 nm.This mciro-convexs is the claw-like structure with one side sharp and the other is flat.Its forward angle is small.The backward angle of the Elaphe rufodoreata and Zaocys dhumnades is small while the one of python is quite great.The density of micro convex on the ventral scale surface of python is the highest of all, which is as 1.8 times as that of the others.There is no plate-like structure on the ventral scale surface of Zaocys dhumnades,the plate-like structure of the Burmese python and Elaphe rufodoreata is 1/2 of the periodicity of terraces.The density of micro-pits on the ventral scale surface of Elaphe rufodoreata's is high,and the one of Zaocys dhumnades is low.The depth and diameter of micro-holes on the ventral scale surface of Zaocys dhumnades and Elaphe rufodoreata are far bigger than their micro-pits.The similar microstructure of the ventral scale surface makes it have the role to decrease the conglutination,less the resistance.The difference of different kind's snakes is to make them to adapt for the outer environment.The water contact angle of the ventral scale surface is measured by water contact angle tester.The result shows that the ventral scale surface snakes are hydrophobic.The mean water contacting angle of ventral scale surface of python,Zaocys dhumnades and Elaphe rufodoreata is 110.6°, 105.8°and 104.5°,respectively.The water contacting angle decreases by 2°to 3°after wetted.The increase of the density and the height of micro convex on the ventral scale surface will enlarge the water contacting angle.The nano mechanical properties of the ventral scale material are researched by using nanoindentation.The result shows that the elastic moduli for ventral scale of the Burmese python and Elaphe rufodoreata are 1310 MPa and 920 MPa,respectively and the nano-hardness are 104 MPa and 90 Mpa,respectively.The mechanical properties of the python's ventral scale are higher than the one of Elaphe rufodoreata.Creep deformation in the ventral scale material occurs during the process of impressing.UMT-2 model of universal micro materials tester is used to research the macro friction properties of the ventral scale surfaces.It is found the friction resistance between ventral scale surface and steel ball is low,and the coefficient is 0.05 to 0.13.The friction coefficient of python's sample is the lowest of all,which is equal to the one of the PTFE,and the one of the Elaphe rufodoreata is the biggest of all.Under dry friction and water lubrication,the friction coefficient decreases as the load or sliding velocity increases.Under oil lubrication condition,the friction coefficient decreases with the increase of the load and the sliding velocity.The friction coefficient under water lubrication is 1.3 to 1.9 times of that under dry friction.And the friction coefficient under oil lubrication is 0.8 to 2.3 times of that under dry friction.The friction force for the ventral scale surface is anisotropic.The forward motion friction resistance is the lowest of all and the backward one is the biggest.The friction resistance in the lateral motion is between the two above,which is nearly equal to the backward one.The friction resistance in backward motion is 1.3 to 1.5 times of that in the forward motion.The lean angle of the micro convex on the ventral scale surface is the main cause of the friction anisotropy.The friction resistance of sliding motion is composed of the resistance due to the adhesion force on the surface,plough force,the mechanical distortion resistance of the micro-convex and elastic resistance,and among them the adhesion force is the main one.The hydrophobia of the ventral scale surface decreases the surface energy.The micro-convex can destroy the continuous water film. The micro-hole on the surface stores the snake's secretion,increasing the surface's hydrophobia.And these are the reasons for decreasing adhesion force and soil's adhesion force function of snake ventral scale surface.The slope angle of microstructure in the ventral scale surface is the main cause of anisotropic friction properties.The nano friction properties of ventral scale surface are researched by using LFM mode of AFM.As a result,the adhesion force and surface energy for ventral scale surface of snake is low.The adhesion energies for ventral scale surface of Burmese python,Elaphe rufodoreata and Zaocys dhumnades are 0.042 J/m~2,0.045 J/m~2 and 0.047 J/m~2,respectively.The mean adhesion force of ventral scale surface decreases as the surface roughness increases.The friction force of ventral scale surface is related to topography and slope of the micro-convex.The periodicity of the friction variation is the same as the one of the micro-convex.It is not related to the height of the micro-convex,but is highly related to the slope of the micro-convex surface,which is the same as the distribution of the micro-convex.At the brink of the micro-convex on ventral scale surface of python,the friction force vary greatly when the tip is ascending phase,and the varying range is twice the one when the tip is drop phase.The friction force of the ventral scale surface linearly increases as the load increases, but not related to the velocity.During the low load,due to the surface adhesion,the micro friction becomes the main factor of the friction.The friction coefficient for the ventral scale of python is the lowest of all,which is basically the same as the one of Elaphe rufodoreata and Zaocys dhumnades.