Reduction of sliding resistance between clay and bionics plates J. Li 1, Z. Cui 1, L. Ren 1, J. Sun 2 & Y. Y. Yan 3 1 Key Laboratory of Terrain Machinery Bionics Engineering, Jilin University, China 2 School of Living Science, Peking University, China 3 Faculty of Construction, Computing & Technology, Nottingham Trent University, UK Abstract The surfaces of soil animals which have evolved various kinds of non-smooth surfaces were studied. By means of microscopes, five kinds of non-smooth shapes were sorted. The dynamic adhesion systems, which formed between the bionics non-smooth surfaces and the wet soil, were analyzed to confirm that the systems of non-smooth surface have discontinuous water film, air pocket, and reduced contact area. The phenomena of contact interface have been observed. To investigate the resistance reduction effects, sliding tests were conducted. Test results indicate that the non-smooth surface plates can reduce resistance from 14.4% to 16.5% for convex type, and from 4.0% to 6.2% for concave type, respectively, compared to the traditional smooth surface. Keywords: biomimetics, soil animal, non-smooth surface, anti adhesion, sliding resistance. 1 Introduction More and more creative abilities of engineers have been opened up to the new territories by biomimetics [1]. Increasing evidence is being provided to indicate that Nature, imaginative by necessity, has already solved many of the problems engineering are with. Nature is a consummate engineer. Animals and plants have found, designed by Nature, what works, what suits, what fits, and most important, what lasts on the Earth.
556 Design and Nature II The phenomena of biological non-smoothness are being recognized by engineering during the process of learning from Nature. Some applications of biomimetic non-smoothness have been successfully practiced, such as the fast speed swimsuit which designed by mimicking the non-smooth sharkskin [2], the self-cleaning paint which is developed by copying the non-smooth surface of lotus leaf [3], and drag reduction plow moldboard which is manufactured by simulating the non-smooth cuticle of dung beetle [4]. The presented paper focuses on the interaction relationship between wet soil and the bionics non-smooth surfaces. The characteristics of body surfaces of several typical soil animals were sorted. The dynamic adhesion systems were studied, which formed between the convex type or concave type of bionics nonsmooth surfaces and the wet soil. The mechanism of sliding resistance reduction of two bionics non-smooth surface was analyzed. In the final part of the presentation, the observed test results of variety on the interfaces were described to confirm that the analysis matched well with the actual circumstance. 2 Classification of biological non-smoothness morphology Most soil animals live in moist soil that is highly adhesive. The surfaces of soil animals have evolved to various kinds of macro- or micro- non-smooth surfaces to decrease resistance and adhesion. Different living surroundings and growing mechanisms make the skins of soil animals of different species, or the same species in different locations, display different rough structures. The body surfaces, skin or cuticle, of some typical soil animals, such as earthworms, ants, ground beetles (Campalita chinese kirby), dung beetles (Copris ochus Motschulsky), mole crickets, loaches, earwigs, pangolins (Manis pentadactyla), locusts, millipedes, centipedes, and field mouse (Spermophilus dauricus) were studied by means of electron-microscopes, stereo-microscope and biomicroscope. A portable video-recorder was used to take pictures of the ways in which the animals contacted soil. To investigate soil animals in greater detail, live animals were collected, and their surfaces were analyzed by using the scanning electron-microscope and stereo-microscope. Five kind of non-smooth shapes were sorted from the analysis works. 2.1 The non-smooth surface with convex protuberances The convex kind of non-smooth surface usually exists in the places where pressure and friction occur between soil and animals. Many small spherical convex protuberances are distributed on its head and front feet. Figure 1 shows the front feet of a dung beetle, taken with the electron microscope. Ground beetles can cut soil with the jaw at the front of the head, but this is usually avoided as they move through gaps in the soil. Because pressure and friction exist between the elytra (forewings) and the soil, many convex protuberances are distributed on the elytra of a ground beetle (fig. 2). The benefit is that the ground beetle is able to move faster in the soil.
Design and Nature II 557 (a) Figure 1: (b) A dung beetle (a) and its front feet (b). Figure 2: Elytrum of ground beetle. 2.2 The non-smooth surface with concave pits It was found that the head dung beetle is full of concave pits (fig. 3). When the dung beetle excavates and pushes soil, the cut soil moves along the non-smooth surface of the "bulldozing plate" and drops loosely on the back plate behind the bulldozing plate. The back plate is the shell between the head and elytra and has many depressions distributed on it. It has been observed that this kind of rough surface is beneficial in reducing resistance and soil shedding. The ground beetle cuts soil with its jaw, and the cut soil drops loosely on the head and the back plate. Many small depressions are distributed on its head and back plate as shown in figure 4. Figure 3: Surface of head of a dung beetle.
558 Design and Nature II Figure 4: Head of ground beetle. 2.3 The non-smooth surface of wavy shape Stepped and wavy shape appear in the belly of soil animals such as the dung beetle, mole cricket, ground beetle. The wavy direction is longitudinal along the motion as shown in a stereo microscope photograph of the belly of a mole cricket (fig. 5). For the earthworm, its skin is also wavy kind as illustrated in figure 6. Figure 5: Belly of a mole cricket. Figure 6: An earthworm.
Design and Nature II 559 Earthworm (fig. 6), millipede, earwig, and centipede are long and thin. Their bodies have adapted to their environment by dividing the total body into many small units or nodes. Each somite has a pair of short feet. Somites can pile up on each other, stretch and draw back, twist, and expand, so it may be considered that the animal appears approximately as a wavy shape along the longitudinal direction. This also can decrease contact area and adhesion, and improve soil shedding. 2.4 The non-smooth surface of scale shape The typical animal of which the body shape is a non-smooth scaly surface structure is the pangolin (fig. 7). Its longitudinal direction of the body appears as a regular wavy curve. The head of ant, and the body surfaces of the loach, earwig, and millipede also have this scaly non-smooth shape. The mechanism of reducing adhesion and resistance is the same as that of the convex protuberances of the dung beetle head. Figure 7: (a) (b) Pangolins (a) and its scale skin (b). 2.5 The flexible non-smooth surface with hairs Many body surfaces of soil animals possess flexibility combined with the geometrical non-smoothness organically [5]. For example, field mice (Spermophilus dauricus) choose to live near water or dump soil. The body surface of field mice (fig. 8a) is coved by hairs and does not adhere to soil at all. There are annular scales on hair shafts (fig. 8b). The scale density is around 10 to 30 rows per 0.1mm. The scale geometric non-smooth structure is made by the scaled hairs. Each annular scale could be taken as a structural unit, and the hair shaft is combined linearly by such units. Tests confirmed that the fur of rabbit and mouse could reduce soil sliding resistance greatly [6]. 3 Dynamic adhesion systems on non-smooth surface The present studies have shown that the surfaces of living things are of nonsmooth morphologies, such as flexible, scaly, wavy, convex protuberances, and concave depressions. On the cross section along the moving direction, most non-smooth surface has the same shape. To make it simple, we studied the dynamic adhesion systems, which formed between the convex and concave type
560 Design and Nature II of bionics non-smooth surfaces and the wet soil. The convex protuberances and concave pits are the non-smooth units. Geometrically non-smooth construction units distributed either regularly or randomly to form a continuous geometrical surface, mechanically affect soil movement and change sliding resistance compared with that for a smooth surface. (a) (b) Figure 8: A field mouse (a) and the SEM structure of its hair (b). Figure 9: The interface in the non-smooth surface. Figure 9 shows the dynamic adhesion system founded in the vicinity of a nonsmooth surface. The main difference between the systems of smooth and nonsmooth surface is that the latter has discontinuous water film (fig.9, section III) and air pocket (fig.9, section V) which makes the soil can not connect to the solid at the rear section of the crown. The phenomenon of the air pocket has been observed through a plexiglass plate with crowns as shown in figure 10, and a glass plate with pits in figure 11, respectively sliding over moisture clay. The factual connect area are also changed for the non-smooth surface adhesion system. Observed variety on the interfaces matched well with the analysis above.
Design and Nature II 561 Figure 10: A rightward sliding plexiglass plate with crowns. Figure 11: A rightward sliding glass plate with pits. 4 The mechanism of sliding resistance reduction When a solid surface and soil slide relative to one another, the frictional resistance of the contact surface must satisfy the equation of Coulomb: F = CaA + Ptanφ Where, Ca = soil-metal adhesion (Pa); φ = angle of soil-metal friction (º); P = normal force on surface (N); F = frictional resistance (N); A = contact area (m 2 ). In adhesive soil, the frictional resistance, F, is mainly produced by adhesion, and can be minimized if the contact area (A) is reduced. Soil animals have
562 Design and Nature II effectively done this by optimizing themselves over millions of years. The longitudinal section of an animal's surface has many small convex asperities that appear as corrugations. When the soil animal moves in the soil, these protuberances contact the soil. If soil contact is avoided in the concave positions, the area of the body surface of the animals in contact with soil is decreased. The concave positions of the body surface of the animals that contain air, reduces both coefficients of friction and adhesion, so the frictional resistance is reduced. The function of the non-smooth surface of concave shape would be in two aspects. First, the concave depressions store air when the cut soil is moving, and the air stored in the depressions can form a gas film between soil and surface, and, secondly, this kind of structure can reduce the soil contact area (A). For the wavy and scale shape, the change of curvature is smooth along the moving direction, normally from head to tail. The resistance would be greater in the opposite direction due to the height difference. The contact pressures at the interface between soil and the cuticle will be different because of the change of cross section. Discontinuities exist at the interface between the soil and skin surface. Then the contact area of the biological surface and soil is reduced and frictional resistance is also decreased. Flexible deformation occurs when the fur contact with soil. While pressured, the hair shaft will show flexibility to absorb some energy. The arrangement of the hairs makes fur kind cuticles become the flexible non-smooth surface, in which every hair is regarded as one structural unit. The arrector pili muscle drives hair shafts move or vibrate to avoid adhesion and reduce resistance. 5 Experiments To investigate the resistance reduction effects, two kind macro- non-smooth plates were made of steel with convex protuberances and concave pits, respectively. For the plate, 200 mm wide and 250 mm length with convex protuberances, the radius of convex protuberances on the plate is 13.5 mm, the height of the convex protuberance is 4 mm, and the interval in the length and wide directions is 40 mm. For the plate, 120 mm wide and 160 mm length with concave pits, the radius of pits on the plate is 9 mm, the depth of the pit is 2.5 mm, and the interval in the length and wide directions is 30 mm. The area ratio of the non-smooth units in the plate is 26.3% for convex plate, and 22.5% for concave plate, respectively. The sliding tests were conducted in the soil bin. Test soil for convex plate is black clay with 27.3% moisture content, and the normal load changes from 30N to 225N; for concave plate, it is yellow clay with 35.1% water content, and load from 33.8N to 86.4N. The slide speed was 0.016 m/s. Figure 12a shows the resistance curves of the convex plate and the smooth plate, figure 12b is the results of the slide resistance of concave plate and smooth plate. Test results indicate that the non-smooth surface plates can reduce resistance 14.4% to 16.5% for convex type, and 4.0% to 6.2% for concave type, respectively, comparing to
Design and Nature II 563 the traditional smooth surface. The resistance reduction phenomena reveals bionics non-smooth surface has a potential benefit in engineering practice. Figure 12: Soil sliding resistance of different plates. 6 Conclusion 1 The surfaces of soil animals have evolved to various kinds of macro- or micro- non-smooth surfaces to decrease resistance and adhesion. Analyzed by the scanning electron-microscope and stereo-microscope, five kind of non-smooth shapes were sorted including the non-smooth surfaces with convex protuberances, concave pits, wavy shape, scale shape, and the flexible non-smooth surface with hairs.
564 Design and Nature II 2 The dynamic adhesion systems, which formed between the convex and concave type of bionics non-smooth surfaces and the wet soil was studied. The systems of non-smooth surface have discontinuous water film, air pocket which makes the soil can not connect to the solid at the rear section of the crown, and reduced contact area. The phenomena of contact interface have been observed through a plexiglass plate with crowns and a glass plate with pits, respectively sliding over moisture clay. 3 To investigate the resistance reduction effects, two kind macro- non-smooth plates were made of steel with convex protuberances and concave pits, respectively. The sliding tests results indicate that the non-smooth surface plates can reduce resistance 14.4% to 16.5% for convex type, and 4.0% to 6.2% for concave type, respectively, comparing to the traditional smooth surface. Acknowledgements The work reported in this paper is supported by China National Key Grant of Basic Scientific Project under grant No. 2002CCA01200, the National Natural Science Foundation of China (Grant No. 50175045), and the Royal Society UK- China Joint Project 2003-2006 (Ref. 15127). References [1] Vincent J.F.V. and Mann, D.L. Systematic technology transfer from biology to engineering. Philosophical transaction of the Royal Society A, 360(1791), 159-173, 2002. [2] Bechert, D.W., Bartenwerfer, M. and Hoppe, G. Drag reduction mechanisms derived from shark skin. ICAS Proc. 15 th Congress of Aeronautical Sciences. Vol. 2, pp. 1044-1068, 1986. [3] Barthlott, W. and Neinbus, C., Purity of the sacred louts, or escape from contamination in biological surfaces, Planta, 202, 1-8, 1997. [4] Ren, L-Q., Tong, J., Li, J-Q. and Cheng, B-C., Soil adhesion and biomimetics of soil-engaging components: a review. J. Agriculture Engineering, 79(3), 239-263, 2001. [5] Ren, L-Q., Wang, Y-P., Li, J-Q. and Tong, J., Flexible unsmoothed cuticles of soil animals and their characteristics of reducing adhesion and resistance. Chinese science bulletin, 43(2), 166-169, 1998. [6] Ren, L-Q., Li, J-Q., Tong, J., Wang, Y-P. and Cong, Q., Applications of the Bionics Flexibility Technology for Anti-adhesion between Soil and Working Components. Proceedings of the 14th International Conference of the Society for Terrain-Vehicle Systems, Vicksburg, USA, October 20-24, 2002.