Jurnal Tribologi 18 (2018) 97-107

Investigations on anisotropic frictional response on different types of shed ’s skin

Jishnu Pradap Pradap 1, Kalayarasan M. 2,*, Muthu Dhanu Shanth 2, Prasanth Kumar T. S. 2, Prasanth A. S. 2, Naveen A. 2, Ilanthirayan Pachaiyappan 2

1 PSG Institute of Advanced Studies, Coimbatore, 641004, . 2 Department of Mechanical Engineering, PSG college of Technology, Coimbatore, Tamil Nadu 641004, INDIA. *Corresponding author: [email protected]

KEYWORD ABSTRACT Optimisation of the texture and topography of surfaces which undergo rubbing or sliding plays a decisive role in minimizing frictional losses in a mechanical system. Furthermore, the incorporation of the ability to exhibit varying frictional behaviour along different directions in surfaces can prove to be especially beneficial in mechanical systems possessing more than one degree of Ventral Region freedom. Nature along with evolution has successfully Fibril provided such a surface structure in squamate . Biomimetic This research is aimed at observing the microstructure of Snake skin the snake skin shed through Scanning Electron Wear Microscopy (SEM) and Atomic Force Microscopy (AFM), Tribology and studying the frictional response of shed skins of three indigenous snake namely, Ophiophagus hannah, mucosa and . This paper also establishes a connection between the frictional behaviour and micro-structure in the . The results confirm the presence of anisotropic friction in the ventral scales of snakes.

1.0 INTRODUCTION The frictional properties always depend upon the microstructure of the specimen that will be subjected to relative motion. Researchers are constantly trying to study and incorporate the microstructure that is naturally available in the plants and . Research has been shifted towards (scaled reptiles) for exploring different microstructures and our paper is also correlated with limbless reptiles which is the snake. The so-called “deterministic surfaces” is

Received 21 January 2018; received in revised form 22 April 2018; accepted 20 May 2018. To cite this article: Pradap et al. (2018). Investigations on anisotropic frictional response on different types of shed snake’s skin. Jurnal Tribologi 18, pp.97-107.

© 2018 Malaysian Tribology Society (MYTRIBOS). All rights reserved.

Jurnal Tribologi 18 (2018) 97-107

termed as the future of surface engineering. For this construction of structured deterministic surfaces, a thorough understanding of the interaction between surface topography, roughness parameters and frictional response are to be studied. Creation of new self-cleaning surfaces, laser textured surfaces and etched surfaces takes great inspiration from surrounding nature. The application of anisotropic frictional properties in locomotion is known to be exhibited by numerous animals. This includes stick insects which achieve frictional anisotropy through their tarsal attachment pads, cockroaches through the webbed ridges on their distal euplantula and the sandfish which moves through desert sand in an undulatory motion (Baumgartner, 2012; Liu et al., 2015; Maladen, 2005). The reptilian skin consists of up to six sub-layers existing as a protective coating for the body. An old epidermal layer of the skin is generally shed as a single piece which consists of four layers: the Oberhautchen, the β-layer, the mesos layer and the α-layer. The oberhautchen layer is the outermost ply containing the micro-textural ornamentation. It is the layer which in direct contact with the surroundings. Also, the exuvium surface geometry of shed epidermis does not differ from that of a live (Klein et al., 2010; Klein et al., 2012). For effective locomotion, the ventral body surface needs to possess anisotropic frictional properties, which can originate from macroscopic structures (Gray et al., 1950; Marvi et al., 2012) such as the overlapping scales. The arrangement of scales provides the possibility of interlocking between their edges and asperities of the substrate. Also microscopic structures of the skin scales, so calledmicroornamentation (Abdel-Aal et al., 2012; Hazel et al., 1999; Renous et al., 1985) and specific adaptations of the material architecture of the skin, like highly ordered embedded fibers which can potentially influence material properties (Politi et al., 2012) might contribute to the frictional anisotropy. Baum et al. (2014a) analyzed the anisotropic microornamentation of ventral body scales using snake-inspired microstructured polymer surface (SIMPS) made of epoxy resin. It is concluded that friction is dominated based on molecular interaction depending on the real contact area and the mechanical interlocking of both contacting surfaces. Klein et al. (2010) observed the gradient in snake skin and found it improving wear resistance against abrasion. Also, materials like human tooth have agradient from a stiff surface to a soft depth making it more wear resistant. Snakes employ various techniques of terrestrial locomotion mainly, lateral undulation or serpentine, slide-pushing, side winding, rectilinear locomotion and concertina locomotion (Gray et al., 1950). Bio-inspired approaches have been shown to yield fruitful results in the case of engineering problems (Gao et al., 2007). Consequently, biomimetics can also aid in devising novel methods to texture surfaces which may lead to modifications in the frictional responses of such deterministic surfaces. Hisham (2013) have studied the feasibility of producing a laser textured surface based on the reptilian morphology. Snake-inspired surface morphologies created through laser texturing have been found to reduce dry sliding friction forces by more than 40% and to increase friction by a factor of three in lubricated surfaces (Lillywhite et al., 2014). Cuervo et al. (2016) developed textured surfaces with low friction based on snake skin and reported that the COF is found to be less than the normal surface for all times. Our work is focused towards the study of three different species of snake’s shed skin for microstructure and frictional properties. In the present work, the species are selected on the basis of their locomotion techniques.

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2.0 MATERIALS AND METHODS

2.1 The king cobra (Ophiophagus hannah) is a venomous snake predominantly found in India and . It is the world's longest venomous snake and can measure up to 5.6 m, typically weighing about 6 kg. King cobras are fast, agile and use lateral undulation for locomotion (Lillywhite et al., 2014).

2.2 Indian Ptyas mucosa is a common species of colubrid snake found in parts of South and Southeast Asia. They grow up to 3 m and are arboreal, non-venomous, and fast-moving. They inhabit forest floors, wetlands, rice paddies, farmlands and they exhibit lateral undulation on planes and concertina for arboreal locomotion (Jayne et al., 2011; Lillywhite et al., 2014).

2.3 Indian rock python Python molurus also is known as the Indian python, is a large non-venomous python species found in many tropic and subtropic areas of the Indian Subcontinent and Southeast Asia. It usually reaches 3 metres in length. It is lethargic and slow moving in nature and exhibits rectilinear motion (Lillywhite et al., 2014).

3.0 MICROSTRUCTURE ANALYSIS

3.1 Skin treatment Three snake skin sheds of each of the snake species are obtained from VOC Park, Coimbatore, India. Each of the snake skin sheds is soaked in distilled water for 10 minutes to remove any surface impurities and is then gently enclosed in paper towels to remove any excess water. Finally, they are cleaned by air blow drying. Skin samples did not undergo any chemical or physical treatment. After that shed skin is kept in air tight container until its testing.

3.2 SEM and AFM analysis Samples of 1x1 cm are taken from the mid ventral section of the skin for both Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis (Kumar et al., 2017). A gold sputter coating is initially applied to the samples to induce conductivity for SEM analysis while no pre-treatment is required for the AFM analysis. SEM analysis is carried out by using ZEISS Evo 18 and AFM using NTMTD, Russia. Figures 1, 2 and 3 shows the SEM image of the three species while Figures 6, 7 and 8 shows AFM images. After that, the images are measured and analysed using ImageJ software.

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Figure 1: Microstructure of King Cobra’s shed Figure 2: Microstructure of Rat Snake’s shed skin. skin.

Figure 3: Microstructure of Indian Python’s Figure 4: Measurement of FAR. shed skin.

In order to arrive at a correlation between locomotion and the fibril structure, various parameters are defined in order to define the geometrical configuration of the fibrils. From the SEM images, the length (l) and width (w) of the fibrils, as well as the distance between adjacent rows of fibrils (b), are determined which is shown in Figure 4. Ten measurements from different positions are obtained for each of the dimensions. Figure 5 depicts the scatter of measurements obtained for the length of the fibril for king cobra.

Figure 5: Scatter plot of fibril length obtained from consecutive trials at different positions.

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The arithmetic mean of these measurements is considered. The Fibril Aspect Ratio (FAR) has been estimated using the equation 1, 푙 퐹퐴푅 = (1) 푤 Where l is the length of individual fibril and w is the width of the individual fibril. By FAR it is possible to compare the relative lengths of fibrils with respect to their widths. The mean values of the obtained measurements are found in Table 1.

Figure 6: AFM 2D & 3D of King Cobra’s shed skin.

Figure 7: AFM 2D & 3D of Indian Python’s shed skin.

Figure 8: AFM 3D & 2D of Rat Snake’s shed skin.

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Table 1: Fibril dimensions. King Cobra Indian Python Rat Snake Fibril Length l (µm) 2.18 ± 0.376 1.04 ± 0.123 1.81 ± 0.214 Fibril Width w (µm) 0.726 ± 0.112 0.299 ± 0.0316 0.522 ± 0.0440 Fibril Aspect Ratio FAR 2.9476 3.4782 3.4674 Adjacent Row spacing b (µm) 1.52 ± 0.381 3.33 ± 0.114 0.859 ± 0.0916

From table 1, that the longest fibril length can be observed for the king cobra. Following king cobra is the rat snake and lastly the Indian python. The widths of the fibrils follow the same order. The closeness of the rows of fibrils to one another as indicated by b follow the decreasing order of King cobra, rat snake and Indian python. The parameters l, w and b also indicate the varying areas occupied by the fibrils of each of the snakes. The number of fibrils per unit area would be the least in case of the Indian python while the comparison between the cobra and rat snake cannot be easily made. Also, fibril density is found too high in the middle section of the trunk. These results are in accordance with the previous literature (Hazel et al., 1999). The fibril aspect ratios followed the descending order: Indian python, rat snake, King cobra. The rat snake and Indian python exhibited very similar fibril aspect ratios despite having different modes of locomotion. This could be attributed to a coincidence and but more comparative data is required for further study. From the AFM analysis, the average heights of fibrils in the mid-trunk section of the ventral scales are determined. For the King Cobra it is between 150 nm and 170 nm, between 190 nm and 215 nm for the Indian Python and 60 to 75 nm for the Rat snake. Hence of the three species, Indian Python had the highest fibril height while the Rat snake had the lowest. The high fibril height makes a large amount of overall contact area making the Indian Python slowest of three. Also obviously the low fibril height makes the Rat snake fastest of three species that are being compared. This inference has been found in correlation with laws of friction as the amount of contact area increases the frictional force required reducing the speed of propulsion of snake.

3.0 FRICTION MEASUREMENT USING EXPERIMENTS Skin samples are taken from the ventral scales which are located in the mid-trunk section of the snake and are analyzed in a pin-on-disc Tribometer (Ducom TR-20-M26) as shown in Figure 9 (Nuraliza et al., 2016). Aluminium pins of 8 mm diameter and 50 mm length are used for testing. As shown is Figure 10, the snake skin is wrapped around the pin such that the fibril is exposed outward. A thin layer of cotton is placed in between the pin and the skin to mimic the snake body for real conditions.

Figure 9: Side view of Tr-20-M26 Pin on Disc apparatus.

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Figure 10: Al Pin wrapped by shed skin.

For each of the skin samples, frictional measurements are performed in four directions. A constant normal force (Fn) of 10 N is applied and speed is kept at 100 rpm. This is an acceptable magnitude of force as all of the three snakes are relatively heavy snakes and it is also known that a higher amount of mass is generally located in the mid-section of the snake compared to other regions during locomotion (Baumgartner, 2012). Three trials for each sample is conducted to obtain the frictional force (Ff) for each of the sliding directions. The sliding directions are denoted by straight forward (SF) that is the direction from the head end to the tail end, straight backward (SB) which is from tail end to head end, lateral forward (LF) indicating measurement along the lateral axis, and conversely lateral backward (LB) (Gao et al., 2007).

Figure 11: Snake motions.

The coefficient of friction (COF) is then estimated using the formula: COF = Ff/Fn. For the relative comparison of COFs in the different directions, the straight forward COF is considered as 100% in all three snakes.

Table 2: COF Values of Rat Snake. Coefficient of Percentage Frictional force(N) friction comparison (%) Forward(SF) 3.283 0.3283 100 Right(LB) 3.617 0.3617 111.53 Reverse(SB) 4.166 0.4166 126.90 Left(LF) 3.591 0.3591 109.38

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Table 3: COF Values of King Cobra. Coefficient of Percentage Frictional force(N) friction comparison (%) Forward(SF) 3.216 0.3216 100 Right(LB) 3.724 0.3724 115.80 Reverse(SB) 4.441 0.4441 138.10 Left(LF) 3.666 0.3666 113.99

Table 4: COF Values of Indian Python Coefficient of Percentage Frictional force(N) friction comparison (%) Forward(SF) 3.066 0.3066 100 Right(LB) 2.872 0.2872 93.67 Reverse(SB) 3.994 0.3994 130.27 Left(LF) 2.578 0.2578 84.08

From the Tables 2, 3 and 4 it is apparent that there is an anisotropy in frictional properties of the shed skin samples. The forward motion (SF) of each snake is taken as a reference and percentage of increase or decrease in COF is arrived.In all of these cases, backward friction is significantly higher than friction in the forward direction (Hisham et al., 2013). Similar to the results, Abdel-aal et al (2011) also found that the friction coefficient for the skin of a Python regius is high in forwarding motion and less in backward motion (Abdel-Aal et al., 2012; Abdel-Aal et al., 2011). It can be inferred from Tables 2, 3 and 4, that the Cobra’s COF varies from 0.32-0.45, the Rat snake’s from 0.32-0.42 and Python from 0.25-0.40. From Table 3, the king cobra’s friction in its right and left directions are 15.79% and 13.99% higher than in the forward direction. During lateral undulation the snake dynamically distributing its weight so that its belly is periodically loaded (pressed) and unloaded (lifted), concentrating its weight on specific points of contact (Hu et al., 2009). This results in an uneven weight distribution that aids in the limbless locomotion. Previous research has indicated the necessity of a minimum of three points of contact between the snake and the substrate during lateral undulation (Gray et al., 1950). Thus the higher values of COF found in the lateral forward and backward directions of the King cobra can be attributed to its locomotion which requires the need to avoid lateral slippage. Rat snake which also follows lateral undulation exhibits a similar pattern of COF to that of the King cobra. It exhibits 11.53% and 9.38% higher COF in its right and left directions respectively in comparison with the forward direction. Its lateral and backward COFs are greater than its Straight Forward (SF) COF. The variation in environment, namely its arboreal nature, forces the rat snake to utilize a different locomotion technique for balancing and gripping. It has also been hypothesized that the arboreal nature of the rat snake could contribute to the variation in its fibril structure (Jayne et al., 2011). On the other hand, the Indian python which employs a rectilinear motion is found to have its lateral COFs lesser than that in the forward direction namely 93.67% and 84.08% in the right and left directions. King Cobra has the highest backward COF as 138% compared to its forward COF. Since the motion is linear, lateral bending of the body and therefore lateral friction does not come into the foreground (Lissmann et al., 1950). The results are in accordance with the previous literature (Berthé et al., 2009; Greiner et al., 2015; Hazel et al., 1999). The data shows the evidence for friction differential on the geometry of the surface which is termed as friction anisotropy. Also, the microstructure shows that the apex of the fibril profile is

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the point that will contact the substrate when the reptile moves. The reason for friction anisotropy is due to that the fibril tips have an asymmetric profile (Abdel-Aal et al., 2011). The slope of the fibril tips is gradual in the forward direction of motion whereas it is steep in the backward direction of motion. This ratchet action also results in a differential friction known by friction anisotropy (Hazel et al., 1999). Hence it can be concluded that there is lesser resistance moving in the forward direction than in the reverse direction. This property is essential to propel the snake forwards. The friction is also affected due to the geometry of the tribo-pair. Hence modification of surface roughness can improve the tribological performance of meso scale contacts as the stick slip is being reduced (Baum et al., 2014b; Sondhauß et al., 2011). By the accepted assumption of a correlation between stick-slip motion and frictional coefficient, the reduced stick-slip motion will have less wear on surfaces (Berman et al., 1996; Urbakh et al., 2004).

CONCLUSIONS In summary, the following conclusions are drawn giving further insight on friction anisotropy in snake skin during different motion. (a) The fibril tips point towards the back of the snake and a shape having a progressive slope. The asymmetric shape enables the snake to have a higher frictional coefficient in the backward direction. (b) From the slope of the individual fibril, the python is found to have the highest slope while the rat snake has the least. (c) The macro friction exhibited by the skins also matches the locomotion requirements of the snakes. Friction is found to be lesser in the lateral directions in Indian python which follows rectilinear motion, wherein lateral motion takes a backseat. (d) And in the case of lateral undulation, where lateral movement plays a significant role, a higher COF is found in lateral directions. (e) The frictional response of the snake skin aids in its locomotion and the microscopic fibrils play a role in the causation of anisotropic friction.

ACKNOWLEDGEMENT The authors would like to thank VOC Park, Coimbatore, Tamil Nadu, India for providing the snake skins to conduct tests.

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