SW—Soil and Water: Soil Adhesion and Biomimetics of Soil-engaging Components: a Review

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    and energy consumption of these machines, but also decreases the quality of work. Characteristics of soil adhesion to

    solid surfaces, behaviour and principles of soil-burrowing animals for improved soil scouring and biomimetics ofsoil-engaging components are reviewed in this paper. The characteristics of soil adhesion to solid surfaces wereconcerned with: the morphological features of soil at the contact interfaces, contact models of soil adhesion,explanation of soil adhesion, factors a!ecting soil adhesion and conventional methods for reducing adhesion. Detailson the behaviour of soil-burrowing animals include: claw shape; body surface geometry, chemical constitution, liquidsecretion and bioelectricity; and body #exing behaviour. The principles of soil-burrowing animals in soil scouringmainly comprise the e!ects of geometrical morphologies and shapes, hydrophobicity, micro-electro-osmotic systems,lubrication and body surface #exibility. Based on these characteristics, biomimetic methods for reducing soiladhesion to soil-engaging components include: biomimetic non-smooth surfaces; modi"cation of soil-engagingmaterials; biomimetic non-smooth electro-osmosis; and #exible components.

    ( 2001 Silsoe Research Institute

    1. Introduction

    The phenomenon of soil adhesion exists when soil is incontact with a solid interface. At the interface, the slidingresistance is dependent on the adhesion, the stress nor-mal to the interface and the angle of friction between thesoil and the interface. By reducing the adhesion andinterface friction angle to values less than the soil cohe-sion and soil to soil friction angle, respectively, the slidingresistance of soil engaging implements is reduced(Hendrick & Bailey, 1982). For tillage equipment, thequality of work is decreased and the energy consumptionis increased due to soil adhesion. Adhesion of soil tocomponents of sowing machines, such as the furrowopener and covering device, decreases the emergence rate

    buckets, bulldozer blades, and self-unloading boxes ofdump trucks, may be decreased by 30}50% due to soiladhesion, and sometimes they cannot work at all. Soiladhesion increases the rolling resistance of vehicles onsoft ground, for example, in paddy "elds, swamps and seabeaches. Excessive adhesion of soil to the running gearcan immobilize the vehicle completely.

    Soil adhesion properties*including the adhesionmechanism, contributory factors and methods for reduc-ing soil adhesion*have been investigated by many re-searchers. Gill and Vanden Berg (1967) and Chancellor(1994) also reviewed the research on soil adhesion andinterfacial friction.

    The problem of soil adhesion has been solved by thesoil-burrowing animals. They have a signi"cant ability toJ. agric. Engng Res. (2001) 79 (3), 239d263doi:10.1006/jaer.2001.0722, available online at http://wwwSW*Soil and Water


    Soil Adhesion and Biomimetics of S

    Lu-Quan Ren; Jin Tong; Jia

    Research Institute for Agricultural Machinery Engineering, Jilin Univere-mail of corresponding

    (Received 30 August 1999; accepted in revised for

    Adhesive forces exist between soil and the surfaces of soincluding tillage and sowing machines. This phenomenonof seeds considerably. The quality of fertilizing and trans-planting is also decreased due to soil adhesion. Tech-niques for reducing soil adhesion are also important inthe harvesting of root crops, such as sugar beet(Vermeulen et al., 1997). The output of soil-engagingcomponents of earthmoving machines, such as excavator

    0021-8634/01/070239#25 $35.00/0 23.idealibrary.com on


    oil-engaging Components: a Review

    n-Qiao Li; Bing-Cong Chen

    sity (Nanling Campus), Changchun 130025, Peoples Republic of China;uthor: jtong@jlu.edu.cn

    m 15 March 2001; published online 6 June 2001)

    l-engaging components on a variety of terrain machinesof soil adhesion not only increases the working resistanceprevent soil from sticking to their bodies because of theevolution of their biological systems through the ex-change of matter, energy and information with soil overmillions of years. They can move in soil without soilsticking to their bodies, especially in clay. Observationof the anti-adhesive characteristics of soil-burrowing

    9 ( 2001 Silsoe Research Institute

  • tanimals led to the study of the principles involved and thepotential application of techniques to soil-engaging com-ponents. The geometric, physico-chemical and mechan-ical features of soil-burrowing animals were utilized todesign soil-engaging materials and structures. This gen-erated the biomimetics of soil-engaging components forreduced adhesion or &anti-adhesion and interfacial fric-tion. This paper reviews soil adhesion, the principles ofsoil-burrowing animals and biomimetics of soil-engagingcomponents in anti-adhesion of soil, and further chal-lenges to biomimetics in this "eld were analysed.

    2. Adhesion of soil to solid surfaces

    2.1. Morphological features of soil at the contactinterfaces and contact models

    2.1.1. Surface morphologies of soil at the contact interfacesThe soil surface morphologies formed at the contact

    interface with solid surfaces were examined by scanningelectron microscopy (Tong et al., 1990a, 1994c). It wasfound that the soil surface cut with a "ne steel wire

    and the dimensions of pores of the surface layer becamesmaller. The topsoil was changed from a loose pattern ofparticle packing to a compact pattern of particle packing,from an unsaturated state to a saturated state, and fromdiscontinuous water menisci to a continuous water "lm.Figures 1(b) and (c) illustrate two typical surface mor-phologies. The roughness of the soil surfaces formed atthe contact interface existed in three sizes: the micro-aggregate; the particle; and the asperity on the particles.The surface feature was dependent upon their topogra-phy and arrangement. Actually, the roughness of soilsurfaces can extend to the molecular scale. The surface ofsoil particles is a molecular fractal surface with a self-similar fractal structure (Pfeifer, 1984; Avnir et al., 1985).The fractal dimension of between two and three, wasa quantitative parameter of the fractal surfaces.

    The surface morphologies of soil taken o! a rubbersurface after air drying were also examined (Tong et al.,1993). It was found that many closed structural unitswere formed at the contact interface [Fig. 1(d )].

    2.1.2. Contact models of soil with solid surfacesOn the basis of the water "lm states and the surface

    morphology at the contact interface, there were twoNota

    a constantb constantD fractal dimensionF tensile force, Nh1

    distance between two solid surfaces, mh2

    distance between two solid surfaces, mJ some property of the interlayer water


    property J of bulk water length of segment, m

    M(r(R) cumulative mass of soil particles, kgM

    Tmass of the total soil particles, kg


    mass of soil, kgm

    wmass of water, kg


    number of embossed convex domesP capillary pressure, PaR characteristic size of particle, m


    disc radius, mR

    Lthe size of the largest soil particle, m


    radius of the theoretical capillary tube, mr soil particle size, m

    LU-QUAN R240displayed a rougher structure [Fig. 1(a)]. When a Te#ondisc was pressed onto the cut soil surface under a certainnormal stress, the plastic #ow of the topsoil took placeion


    ratio of areast time, s< volume content of reinforcement in

    a composite materiala tilt angle, degb constant


    surface tension of water, Nm~1g viscosity of liquid, Pa s


    contact angle of advancing water, deghR

    contact angle of receding water, degh0

    inherent contact angle of water on sur-face, deg


    contact angle of water on matrix mater-ial, deg


    contact angle of water on reinforcingmaterial, deg


    apparent contact angle of water onrough surface, deg


    contact angle of water on compositesurface, deg

    EN E A .contact situations at the interface (Tong et al., 1994c).When the moisture content of soil or the normal stressapplied to the interface was low, a continuous water "lm

  • oFig. 1. Photographs by scanning electron microscopy of the morphc) contacting with a disc of Teyon, and (d) contacting with a rubbe

    SOIL ADHESION A27)1% d. b. and (d) supersaturated b

    was not formed at the interface and there exist "vepossible contact states: completely non-contacting as-perities; water point contacting asperities; water meniscicontacting asperities; water menisci contacting particles;and local water "lm contacting micro-aggregates. Whenthe moisture content of the soil or the normal stress washigh, the contact interface was "lled with water and thesoil was linked with the solid surface by a continuouswater "lm.

    2.2. Explanations of soil adhesion

    2.2.1. Moisture tension of the interfacial water ,lmFountaine (1954) demonstrated that water "lm plays

    a dominant role in soil adhesion. When a continuouswater "lm is formed between a saturated soil and a solidsurface and reaches equilibrium, the moisture tensionwould come to the same value through the soil massincluding the contact interface. In this condition, theshapes of all the water}air menisci satisfy the equation:




    where cLV

    is the surface tension of water, RT

    is the radiusof the theoretical capillary tube and P is the capillarypressure. The soil adhesion as a force per unit area causedby the water "lm is equal to the moisture tension. Fromlogies of soil surfaces formed after (a) cutting with a steel wire, (b,r surface, using (a, b, c) yellow clay soil with a moisture content of

    241ND BIOMIMETICSlack soil (Tong et al., 1993, 1994c)

    Eqn (1), the adhesion is very strong when the water "lm isvery thin and vice versa.

    2.2.2. Molecular attraction and negative air pressureThere exists molecular attraction between the solid

    surface and non-contacting asperities or water point con-tacting asperities of soil, but the molecular attractionwould be strong only if the separation between themreaches the equilibrium intermolecular distance. The mo-lecular attraction can contribute markedly to the soiladhesion. If their separation is very much larger than theequilibrium intermolecular distance, the molecular at-traction force would be negligible for the soil adhesion.There may exist a very large force of molecular attractionbetween a rubber surface and the wall of the closedstructural units in the soil surface layer shown in Fig. 1(d )because the separation reaches the equilibrium inter-molecular distance. In addition, the negative air pressureexisting within the closed structural units may also playan important role. In this condition, the molecular at-traction and the negative air pressure are the main rea-sons for adhesion of soil to rubber.

    2.2.3. Capillary attraction from the interfacialwater ,lm

    Akiyama and Yokoi (1972a, 1972b) represented soilparticles in the geometrically simpli"ed form of spheres

  • with the same size. They analysed the soil adhesion forthe continuous water "lm and water menisci contactstates, respectively, for the ideal soil with the close pack-ing and with the loose packing situations. They con-sidered that the water "lm capillary pressure was thesource of the soil adhesion for the continuous "lm con-tact state; and that the capillary pressure and the surfacetension were the sources of soil adhesion for the watermenisci contact state. Actually, both the moisture tensionand the capillary pressure are due to the surface tensionof the continuous water "lm or water menisci and, there-fore, the conclusions by Fountaine (1954) and byAkiyama and Yokoi (1972a, 1972b) were similar. Bothindicated the important action of the water surface ten-sion on soil adhesion.


  • lele


    NTabAdvancing contact angles hA and receding contact ang

    Solid materialAdvancing cont


    ), de

    Polytetra#uoroethylene (Te#on) 109Polystyrene 91Polyethylene 96Polypropylene 108Para$n wax 110Polycarbonate 84Human skin 90Gold 66Platinum 40Plain carbon steel 73Enamel 29

    of a soil is a fractal, then the log}log plot of M(r(R) andR/R

    Lis a straight line. The fractal dimension D can be

    calculated from the slope of the straight line. Generally,the fractal dimensions of particle size distributions ofsoils are increased with the clay particle content and,therefore, the soil adhesion and frictional force are in-creased (Tong et al., 1994a, 1999; Yan et al., 1996). Themolecular fractal dimension of soil particle surfaces isalso an important factor a!ecting soil adhesion. Thelarger the molecular fractal dimension, the larger is thespeci"c surface area, and the higher the soil adhesion aswell.

    2.3.2. E+ects of properties of soil-engaging componentsurfaces

    The atomic or molecular structure of surfaces of a solidor a liquid is di!erent to that of their bulk materials

    SOIL ADHESION Abecause the structural symmetry of atoms or molecules islost and, therefore, physical and chemical properties ofsurfaces are changed. This has resulted in such surfacephenomena as physical adsorption, chemical adsorption,chemical reaction and soil adhesion as well. On the basisof the surface free energy, surfaces may be classi"ed intohigh-energy surfaces and low-energy surfaces, corre-sponding to high surface energy materials and low sur-face energy materials. High surface energy materialsinclude metals, metallic compounds and inorganic com-pounds (i.e. oxides, nitrides, silica and diamond) and theirsurface tension is from 0)2 to 0)5 Nm~1. Low surfaceenergy materials include organic compounds and organicpolymers, and their surface tension is, mostly, less than0)1 Nm~1. Water is a low surface energy material andhas a surface tension of 0)0728 N m~1 at 203C. High-energy surfaces are hydrophilic and low-energy surfacesare hydrophobic. The hydrophilicity or hydrophobicityof a surface can be expressed by the contact angle ofwater on the surface. The higher the contact angle1s hR of water on the surfaces of some solid materials

    ct angle Receding contactangle (h

    R), deg Reference

    106 Wu (1982)84 Wu (1982)62 Wu (1982)

    Adamson (1976)Adamson (1976)

    68 Wu (1982)Adamson (1976)Adamson (1976)Adamson (1976)

    0 Tong et al. (1994c)0 Tong et al. (1994c)

    of water on the surface, the higher is the hydrophobicityof the surface, that is, the lower the hydrophilicity of thesurface. Table 1 lists advancing contact angles h


    receding contact angles hR

    of water on the surfaces ofsome solid materials. Mostly, the adhesion between soiland hydrophobic materials is poor, that is to say, thelarger the contact angles, the lower is the soil adhesion.Therefore, metallic materials, metallic compounds andinorganic compounds display higher soil adhesion, andpolymeric materials have a lower soil adhesion.

    Li et al. (1993a, 1993b, 1996b) tested the e!ects ofmicrostructure on the adhesion of soil to steel-35. It wasfound that the soil adhesion to steel-35 with a temperedmartensite microstructure and with a tempered sorbitemicrostructure was lower than that of the microstructurewas obtained through quenching and low-temperaturetempering treatment, the tempered troostite through the

    243D BIOMIMETICSquenching and average temperature tempering treat-ment, and the tempered sorbite through the quenchingand high-temperature tempering treatment. Figure 2 il-lustrates the relationship between soil adhesion and thetempering temperature for steel-35. Soil adhesion tosteel-35 is higher within the average tempering temper-ature range.

    2.3.3. E+ects of experimental conditions and workingsurroundings

    Factors a!ecting soil adhesion include such experi-mental conditions and working surroundings as the ap-plied normal stress, the application time for that stress,temperature, electric "eld, magnetic "eld and vibration.Sliding resistance is a monotonically increasing functionof normal stress for most solid materials (Zhang et al.,1986). Sometimes, the soil adhesion was increased toa certain value and then decreased as the normal pressurewas increased (Li et al., 1993b), possibly for the reason ofthe interfacial lubrication by water squeezed out of the

  • 2.4.2. Soil-engaging component materialsFig. 2. Soil adhesion of steel-35 after tempering at various tem-peratures (Li et al., 1996b)

    soil. The surface tension of water is a function of temper-ature. The surface tension of water is decreased as thetemperature is elevated, and so is the viscosity of water.Therefore, an increase of temperature can make the sur-face tension of the interfacial water "lm decrease and, asa result, the soil adhesion can be reduced. When anelectric "eld generated by a negative pole and positivepole is applied to soil, the soil water can move from thepositive pole to the negative pole. This is called theelectro-osmotic phenomenon of soil. If the non-scouringareas of soil contact are given negative polarity, thethickness of the water "lm increases owing to electro-osmosis, which can reduce the soil adhesion (Shen, 1982;Cong et al., 1990a). The nature and properties of the soilcan be changed under a strong magnetic "eld, for

    LU-QUAN R244example: the variable negative electric charge and thepermeability are increased; and the positive electriccharge and the expandability are decreased (Zhuanget al., 1992). These changes, in general, reduce the ad-hesion of soil to solid materials (Han & Zhang, 1991;Zhang & Han, 1992; Guo & Liu, 1995). Ultrasonic vibra-tion and mechanical vibration can reduce the soil ad-hesion and interface friction or separate the adhered soilfrom the solid surfaces (Sharma et al., 1977; Wang et al.,1993, 1996).

    2.4. Conventional methods for reducing soil adhesion

    2.4.1. Surface shape designImprovement of the surface design of soil-engaging

    components is one of the conventional methods for re-ducing soil adhesion and interface friction. This methodis, mostly, based on the kinematic and dynamic analysisof the interactive relation between soil and soil-engagingcomponents. Some convex corrugations made on theFig. 3. Schematic diagram of *comet-type+ oblique hole drilledthrough a plough mouldboard (Zhu et al., 1992)

    soil-contacting surfaces can decrease the real contactarea and break the continuity of the interfacial water "lmbetween soil and solid surfaces, which results in a reduc-tion of the soil adhesion and sliding resistance againstsolid surfaces (Cong et al., 1990b). Zhu et al. (1992)reported a plough with some &comet-type oblique holesdrilled through the ploughshare and the breast of themouldboard. Figure 3 illustrates the schematic diagramof these holes. It was shown that the ploughing resistancewas reduced by 2)5}3)5% and by 8}12% in a moist "eldsoil and under paddy "eld conditions, respectively.

    EN E A .Modi"cation of soil engaging component materials isan important method for reducing soil adhesion andinterface friction. Many researchers have examined thesoil adhesion characteristics of polymeric materials, ow-ing to their lower surface energy. The tested polymericmaterials included Te#on (polytetra#uoroethylene*PTFE), ultra high molecular weight polyethylene(UHMWPE), polyethersulphone}polytetra#uoroethy-lene (PES}PTFE) coating and others (Fox & Bockhup,1965; Salokhe & Gee-Clough, 1989; Tong et al., 1990b,1994c; Lu et al., 1996; Liu et al., 1998b). The applicationsof polymeric materials for soil-engaging componentswere limited due to their poor abrasion resistance againstsoil (Tong et al., 1994b, 1999).

    Salokhe and Gee-Clough (1988, 1989) and Salokheet al. (1990, 1993) examined the soil adhesion character-istics of enamel coating. It was found that the enamelcoating has the ability to reduce soil adhesion. Rollingresistance of cage wheels and the plough boat can bereduced owing to the enamel coating on the wheel lugsand on the boat surfaces and their tractive performance

  • Sharma et al. (1977) tested the e!ect of ultrasonicvibration on the friction of metal against soil. The vibra-tion frequency used was 10 kHz. Wang et al. (1993, 1996)designed a mechanism used for testing the e!ect of mech-anical vibration on soil adhesion. The vibration wasproduced by blowing compressed air. The soil used forthe tests was a silty loam with a moisture content of38)7% d.b. Vibration at frequencies of between 60 and100 Hz was remarkably e!ective in removing soil fromthe plate (Fig. 4).

    2.4.6. ubricationAraya and Kawanishi (1984) and Schafer et al. (1977)

    studied the e!ect of the #ow of air, water and polymer}water solution on the friction between soil and solidsurfaces. The #uids injected between soil and soil-engag-ing components had a lubricating action and, therefore,could cause the draught reduction of soil-engagingwas improved. However, the spalling rupture of the en-amel coating easily took place under the abrasive wearconditions (Tong et al., 1998). The abrasion resistance ofenamel coating may be increased by modifying the rarematerials for the coating and by improving the prepara-tion techniques.

    Cast iron materials are most commonly used forsimple soil tillage implements. It was found that whiteiron, one of the conventional plough mouldboard mater-ials, containing more phosphorus and silicon, improvedthe anti-adhesive properties (Tong et al., 1990c; Chenet al., 1995a). However, it was noted that an excessivephosphorus content tended to increase the brittleness ofwhite iron, limiting the application of phosphoric whiteiron to soil-engaging components.

    2.4.3. Electro-osmosisThe electro-osmotic e!ect is in#uenced by: the soil

    moisture content; soil texture; electric voltage and con-tact time for electro-osmosis; the separation between thezones of positive and negative polarity and their arearatio; and the contact pressure as well. A high voltage forelectro-osmosis and a long contact time are required inorder to increase the thickness of the interfacial water"lm between the soil and the zone of negative polarity.The soil adhesion to the zone of negative polarity couldbe reduced by shortening the separation between the twoterminals. The electro-osmotic e!ect is better for clay soilthan for sandy soil (Cong et al., 1990a, 1990b). Theapplication of the electro-osmotic method to a slidingcomponent in contact with soil was limited because of thelong contact time required for electro-osmosis. Themethod of electro-osmosis may be applied successfullyonly to reducing adhesion of soil to the soil-engagingcomponents with a long contact time between them, forexample, excavator buckets (Cong et al., 1990b). Muchmore energy is consumed when high voltage is used forelectro-osmosis and further research is required into thee!ective use of lower voltages. It was reported thatthe voltage can be decreased by changing the position ofthe zones of polarity and their arrangement (Cong et al.,1990b). Clyma and Larson (1991) also reported that thevoltage could be decreased to 45 V.

    2.4.4. Magnetic ,eldsHan and Zhang (1991) and Guo and Liu (1995) tested

    the e!ect of a magnetic "eld on ploughing resistance.Several permanent magnets were attached to the back ofthe ploughshare. Han and Zhang (1991) reported that theploughing resistance and the fuel consumption of thetractors used for pulling the ploughs were reduced by

    SOIL ADHESION A10)55% and 11)30%, respectively, for the magnetizedplough as compared to the non-magnetized ploughunder the identical "eld conditions (in a medium loam,Fig. 4. Soil separation versus frequencies of vibration showingthe ewects of vibration on scouring of soil-engaging components(Wang et al., 1996); soil spraying speed during preconditioning:

    , 5)0}5)9 m s!1; , 6)0}7)0 m s!1

    with a moisture content of 15)98% d.b.). Guo and Liu(1995) reported that the fuel consumption of the tractorscan be reduced by 7}15% for the magnetized plough ina "eld soil with a moisture content of 12)26}14)28% d.b.and for a ploughing depth of 25}30 mm.

    Zhang and Han (1992) tested the e!ect of the magnetic"eld on the forward resistance of the covering blade ina medium loam soil at a moisture content of 12% d.b.The forward resistance of the magnetized covering bladecan be reduced by 15)4}23)3% as compared with theconventional one with the same dimensions. It was alsofound that the largest reduction in the forward resistanceoccurred at a magnetic induction of 23 mT.


  • LU-QUAN R246Fig. 5. Some special macroscopic feature

    was the additional equipment required to reduce soiladhesion and interfacial friction.

    2.4.7. Additional mechanismsAn additional mechanism can be attached to remove

    the soil adhering to the soil-engaging components (Conget al., 1990b). This method was applied mostly to exca-vator buckets and blades. Ren et al. (1990d) designeda bucket with a #exible #oor and demonstrated that itcould reduce soil accretion in the bucket by 59)4% andloading resistance by 15)85%. Yin et al. (1990) designeda tongue scraper mechanism for a loader bucket to re-move the soil accretion in the bucket. The shovel withthis mechanism was tested in handling soil and powderedcoal and was found to increase loader productivity by20%.

    3. Characteristics of soil-burrowing animals

    3.1. Evolution of soil-burrowing animals

    Soil-burrowing animals include those soil animals liv-ing mainly in the soil and other animals, such as theEN E A .s of soil animals in evolution (Xin, 1986)

    nocturnal pangolin, digging a large subterranean cham-ber or cave in the earth as a den. The living surroundingsof soil animals are di!erent to those of animals on landand in water. Soil is a semi-solid material in which it isdark, moist and di$cult for animal movement. However,soil animals have ability to adapt to the soil surroundingsbecause of their evolution in biological systems overmillions of years. Two di!erent adaptations for soil haveoccurred in soil animals, the active adaptation and thepassive adaptation. Some features of soil animals inevolution are shown in Fig. 5. The stronger digging legswere as a result of active adaptation for burrowing, forexample, the forelegs of mole crickets (Gryllotalpidae) andthe claws of black ground beetles (Carabidae), dung beetles(Scarabaeidae) and Cydnidae (Xin, 1986). The passive ad-aptation of animals to the soil habitat resulted in shorteror vestigial additional legs, the body becoming smaller,thinner or #atter, and wings and eyes diminishing.

    As there is continuous interaction between the soilanimals and the soil in which they move, subsistencewould be decreased and existence threatened if soil couldnot be removed from body surfaces. The anti-adhesivecharacteristics of their body surfaces are an inevitableoutcome of their evolution over millions of years.

  • N3.2. Geometry of body surfaces

    3.2.1. Basic concepts of surface uniformityRen et al. (1992b) described the basic concepts of

    surface texture with respect to soil adhesion, de"ningsmooth and non-smooth surfaces. A surface texturemeeting the living requirements of animals or the work-ing conditions of soil-engaging components is calledsmooth when there exists no surface irregularities whichadversely in#uence the tangential resistance to move-ment. If there exist some construction units, such asconvex domes and dimpled concavities arranged ona smooth surface, the surface can be considered as a geo-metrically non-smooth surface. Geometrically non-smooth construction units, distributed either regularly orrandomly to form a continuous geometrical surface,mechanically a!ect soil movement and change slidingresistance compared with that for a smooth surface.

    Surface irregularities can occur not only by varying thesurface texture but also by using materials of di!erentchemical composition and electrical conductivity andthrough dynamic e!ects. Composite materials can causenon-uniform wetting behaviour. Surface characteristicsaltering from smooth to non-smooth as the rate of shearincreases are called dynamically non-smooth surfaces, astypi"ed by the skin of a dolphin which has a smoothcuticle surface with many hard "bro-nodules underneath.When a dolphin is moving, the "bro-nodules becomeharder and bigger to produce a geometrically non-smooth surface.

    3.2.2. Morphologies of body surfacesAll soil-burrowing animals have geometrically non-

    smooth structures or rough structures on their bodysurfaces. Chen et al. (1990b), Ren et al. (1990c, 1992a),Cong et al. (1992) and Tong et al. (1994d) observed andexamined the geometrical surface morphologies of earth-worm (umbricidae), Oniscidae, Diplopoden, centipede(Chilopoda), dung beetle (Scarabaeidae), ground beetle(Carabidae), ant (Formicidae), mole cricket (Gryllotal-pidae), abidura riparia, cockroach (Blattaria), cricket(Gryllidae), and pangolin. There exist di!erences betweendi!erent species of soil-burrowing animals in the shapeand the size of the non-smooth construction units. Ac-cording to the shape of the unit, the geometrically non-smooth surfaces can be classi"ed as embossed (with smallconvex domes), dimpled (with small concave hollows),wavy, scaly, corrugated (with ridges), stepped and seta-covered; according to the size, there are macroscopic andmicroscopic non-smooth surfaces; and according to thearrangement of the units, there are regular and random

    SOIL ADHESION Ageometrically non-smooth surfaces. Figure 6 shows somephotographs of typical morphologies of body surfacesof soil-burrowing animals and pangolin scales. Thenon-smooth construction units may be rigid, elastic or#exible.

    3.2.3. Shapes of clawsRen et al. (1990b) observed and examined the shapes

    and surface morphologies of claws of the mole cricket,ant, dung beetle and pangolin. It was found that the forepart of the claws of these animals is wedge shaped. Thefore parts of the claws of the ant and pangolin are taperedwedges with a wedge angle of 203, those of the molecricket are square wedges with a wedge angle of 303, andthose of the dung beetle are #at wedges with a wedgeangle of 503. The extent of the claw wedge angle ofsoil-burrowing animals has a certain relationship withthe claw depth for dislodging soil particles and with theanimal habitat, i.e. the properties of the soil. The curva-ture is largest at the claw tip and becomes smaller andsmaller with the distance from the claw tip. There areridges, furrows and some cilia on the surfaces of soil-burrowing animals claws. Figure 7 shows scanningelectron microscopy photographs of claws of a molecricket.

    3.3. Chemical composition of body surfaces

    It was determined by Xu et al. (1990), Tong et al.(1994d, 1995) and Cui et al. (1990) that the cuticle of thedung beetle, black ant, mole cricket, ground beetle andpangolin scales contains carbon, hydrogen, nitrogen,oxygen and some trace elements, such as potassium,chlorine, calcium, silicon, phosphorus, sulphur, sodium,magnesium, aluminium, manganese and iron. The rela-tive contents of the trace elements vary from animal toanimal and from one part to the other parts of the sameanimal. A living body contains, generally, 60}70% ofwater and some organic substances including organiccompounds. Organic substances in the living body aremainly protein, nucleic acid, saccharine and lipoid.Organic compounds include amino acids, fatty acids,glucose, hormones, vitamins, glycerine, urea, uric acid,creatine and some other organic acids. The protein isa high molecular compound and mainly consists of car-bon, hydrogen, oxygen and nitrogen, sometimes, a smallamount of trace elements. The outermost surface layer ofsoil animals cuticle consists of a material similar to resinor paint and possesses a strong hydrophobic property.The contact angles of water on the surfaces of soil ani-mals cuticle are more than 903. Pangolin scales mainlyconsist of a-keratin and b-keratin, which contain 18

    247D BIOMIMETICSamino acids (Tong et al., 1995) (Table 2). The e!ects ofamino acids on the anti-adhesion of soil animals are notcurrently known.

  • LU-QUAN R248Fig. 6. Some characteristic non-smooth morphologies of body ssmall convex domes on the head of a dung beetle; (b) surface dim(c) ridged surface on the abdomen of a ground beetle; (d) steppe

    (Tong et al., 1

    3.4. iquid substance on body surfaces

    3.4.1. Methods for collecting secretion liquid of earthwormThe earthworm is a typical animal covered with

    a liquid secretion on its body surface. In order to examineits chemical composition and anti-adhesive properties,Chen et al. (1990a) tested three methods for collecting theliquid secretion of earthworm (Eisena foetida), the electri-cal stimulation method, the transference method and theimmersion method. For the electrical stimulationmethod, two electrodes were attached to the body andthen the earthworm was stimulated with electricity sothat the liquid substance was secreted. For the transfer-ence method, a layer of food in a glass jar was separatedby a steel wire gauze from an upper layer of clean dryEN E A .urfaces of soil-burrowing animals: (a) surface embossed withpled with small concave hollows on the head of a dung beetle;

    d surface on the head of a black ant: and (e) a pangolin scale994d, 1995)

    sand. Some earthworms were laid on the dry sand for24 h and the liquid substance secreted by the earthwormswas transferred onto the sand grains. The sand grainswith the liquid secretion were placed into a diluted salinesolution and, 24 h afterwards, sand grains and earth-worm casts were "ltered. The solution was desalinatedand then concentrated to obtain the enriched secretionsubstance. For the immersion method, clean earthwormswere immersed into a saline solution with a concentra-tion of 0)2% (w/w) for 24 h. Thereafter, the earthwormswere dredged up and their casts "ltered from the solu-tion. The solution containing the secretion substance wasdesalinated, then concentrated and the enriched secretionsubstance obtained as for the transference method. Chenet al. (1990a) demonstrated that any of these three

  • NSOIL ADHESION AFig. 7. Photographs showing shapes and topography of claws of a(Ren et a

    methods could be used for collecting the secretions ofearthworms.

    3.4.2. Chemical composition of earthworm secretionsLi et al. (1990) examined the chemical composition of

    the liquid secretion of the earthworm (Eisena foetida).The secretion consisted of water (99)71% by weight) anda little of an organic substance with a molecular weight of30 000}50 000. Some 59% of the organic substance wasprotein containing 18 amino acids and the rest was theconjugated saccharide and a little of other water-solublesubstances. The protein was combined with the con-jugated saccharide to yield a conjugated protein, whichwas a mucoprotein. The water-soluble substances weremainly the metabolite, such as ammonia, urea and uricacid coming from the body of the earthworm and organicsalt from the soil. Table 3 gives the amino acid composi-tion of the secretions, the whole body of the earthworm(Eisena foetida) and the secretions of the loach (Misqur-nus fossilis).249D BIOMIMETICSmole cricket: (a) fore claw, (b) middle claw, and (c) hind clawl., 1990)

    3.5. Bioelectricity of body surfaces

    3.5.1. Bioelectrical phenomenonVarious vital activities in nature are accompanied by

    electrical phenomena, that is to say, there exists bioelec-tricity in all living creatures. There are two basic voltagesof bioelectricity, the resting potential and the actionpotential. The surface potential of a living body is a com-bined re#ection of the resting potential and the actionpotential in the body. The resting potential is the poten-tial di!erence between the outside and the inside of thetissue or cell membrane of a living creature when it isresting. The action potential is the potential di!erencebetween the excited part and the resting part of the sametissue or cells. The action potential is of short durationand reversible, and larger than the membrane potential.The resting potential and the action potential of muscleand nerve cells are 60}90 and 90}120 mV, respectively(Ma, 1984). Yamashiro (1989, 1995) reported thepiezoelectric e!ect of a crab shell, lily and bamboo leaf.

  • Table 2Amino acid composition of pangolin scale, hair and feather

    (Tong et al., 1995)

    Keratin content, % by weight

    Amino acid Pangolin scale Hair Feather

    Glycine 10)6}13)7 5)2}6)5 7)2Alanine 2)3}3)0 3)4}4)4 5)4Valine 3)2}4)0 5)0}5)9 8)8Leucine 7)8}7)9 7)6}8)1 8)0Isoleucine 2)0}2)3 3)1}4)5 6)0Serine 5)0}5)5 7)2}9)5 14)0Threonine 3)2}3)4 6)6}6)7 4)8Tyrosine 17)2}20)0 4)0}6)4 2)2Phenylalanine 3)0}3)2 3)4}4)0 5)3Cysteine 1)9 * *Cystine * 11)4}14)1 8)2Methionine 0)4}0)6 0)5}0)7 0)5Tryptophan 0)4}0)5 1)8}2)1 0)7Arginine 8)0}8)6 9)2}10)0 7)5Histidine 0)7}1)2 2)8}3)3 0)7Lysine 2)5}2)6 0)7}1)1 1)7Aspartoyl 7)1}7)9 6)4}7)3 7)5Glutamic acid 10)4}10)9 13)1}16)0 9)7Proline 8)6}9)7 5)8}8)1 10)0

    LU-QUAN R2503.5.2. Surface electric potential of earthwormsSun et al. (1991) measured the surface potential of the

    fore part, the middle part and the hind part of earth-worms (Michaelsen) using non-polarized silver/silverchloride electrodes and silk electrodes. Figure 8 showsa manner of "xing the electrodes to the skin of theearthworm. The measuring electrode was a half ring. Itwas found that the resting potential of the earthwormtended to zero with respect to the earth. When it wascreeping, the skin of the earthworm possessed a negativepotential at the moving body part with respect to theearth and the resting part. The maximum amplitude ofthe surface potential was 40 mV, occurring at its forepart. A circular non-polarized silver/silver chloride elec-trode was also prepared and "xed inside a 10 mm longtube. The inside diameter of this tube was a little less thanthe cross-sectional size of the earthworm tested. Theearthworms were guided through the tube and the sur-face potential was recorded. In this case, the maximumamplitude of the surface potential was 35 mV at the forepart, 19 mV at the middle part and 18 mV at the hindpart. Table 4 gives the average potential measured.

    3.6. Flexibility of soil-burrowing animals

    The #exible features of the skin of pangolin (Manispentadactyla linnacus), vole (Microtas manderinns) andCylindroiulus teutonicus and others were analysed by Renet al. (1996, 1998). Pangolin is a typical soil-burrowinganimal of 700}900 mm in body length and often diggingsubterranean chambers up to 10 m in length. In China,the pangolin lives in wet woodlands in an area of red soilwhich is very cohesive and sticky. The body of the pan-golin is covered with a layer of scales, some with corruga-tions regularly arranged on the scale surface [Fig. 6(e)].Pangolin scales also exhibit a #exible feature by turningupwards and whipping under the muscular action of thedermis beneath the layer of scales. A vole lives in wetareas, such as low-lying land, contour furrows and paddy-"eld borders. The skin of the vole is covered in hair,including needle hair and villi, which has been keratinized.The density of the vole hair is 80}100 per mm2.

    Many soil animals such as the centipede (Chilopoda),Cylindroiusus teutonicus, earthworm and the larvae ofmost insects adopt a creeping mode of movement, pro-gressing forwards on expansion and contraction of theirmusculus longitudinalis and musculus transversus. Thebody of Cylindroiusus teutonicus is circular and has25}100 segments. Two double paraeiopods protrudefrom the abdomen of the Cylindroiusus teutonicus, withstrong intersegmental membranes and cuticle of hardchitin. On the skin of most soil animals, including thecricket, dung beetle and ground beetle, setae or cilia exist.

    A scale, segment, seta or a cilium can be considereda construction unit of their body skin (Ren et al., 1996).These units all possess a #exible motion. The geomet-rically non-smooth surface formed by #exible construc-tion units was called the #exible non-smooth surface andthe corresponding #exibility was called the non-smooth#exibility. In other words, the #exibility of the skin of soilanimals is brought about through their geometricallynon-smooth construction units.

    4. Principles of anti-adhesive characteristics of soilanimals

    4.1. Geometrical morphologies and shapes

    Ren et al. (1990a), Tong et al. (1994d) and Jia et al.(1995b, 1996a) analysed the e!ects of the microscopicgeometrically non-smooth or rough structure of bodysurfaces of soil animals on their wetting behaviour. Thecontour of the non-smooth body surfaces of soil animalswas assumed as a wavy pro"le. A model for the non-smooth body surfaces of soil animals (non-smooth orrough) is shown in Fig. 9. The apparent contact angle

    EN E A .hafor the corrugated surface could be expressed as

    cos ha"r



  • Table 3Amino acid composition of the secretion substances and the whole body of Eisena foetida, of the cuticle of Lumbricus terrestis and the

    secretion substances of the loach Misquirnus fossilis

    Composition, % by weight

    Amino acidSecretion fromEisena foetida

    Whole body ofEisena foetida

    Cuticle of Lumbricusterrestris

    Secretion ofMisgurnus fossilis

    22)39 4)898)82 8)272)46 6)704)27 10)31

    251SOIL ADHESION AND BIOMIMETICSGlycine 6)68 5)39Alanine 5)85 6)55Valine 5)85 3)82Leucine 7)10 8)05Isoleucine 5)22 4)95Serine 5)43 4)61Threonine 9)60 4)98Tyrosine 3)34 4)08Phenylalanine 3)55 4)43Cysteine 1)04 *Cystine * 1)42Meithonine 3)76 0)75Tryptophan 1)25 1)11Arginine 3)97 8)49Histidine 0)84 2)45Lysine 3)34 8)02Aspartoyl 10)44 10)00Glutamic acid 11)27 15)61Proline 11)48 3)32

    HydroxyprolineFree amino acid * 1)98

    Fig. 8. Schematic diagram showing a method for measuring sur-face electric potential of earthworm body (Sun et al., 1991)

    where h0

    is the inherent contact angle of the surfacesubstance and r

    ais the ratio of the actual surface area to

    the projected surface area of the corrugated surface.From Eqn (5), h

    ais larger than h

    0for the body surfaces of

    soil animals because the inherent contact angle of wateron their body surfaces is over 903. It was concluded thatthe geometrically non-smooth morphology of soil ani-mals would enhance their inherent hydrophobicity. Inaddition, when the absolute value of the tilt angle a ismore than (180}h

    0), a composite interface would be for-

    med and air would be intercepted in the troughs. Thelarger was the absolute value of a, the larger h


    If the rough morphology of body surfaces of soil ani-mals are molecular fractals, the apparent contact anglesof water on the fractal rough surfaces are also larger than2)22 6)2910)63 5)047)08 5)340)72 *2)19 5)37



    3)67 2)970)09 0)642)23 4)06

    9)5011)62 16)151)61 11)4720)0*

    Table 4Mean and maximum values of the surface electric potential of

    earthworm Michaelsen body tissue (Sun et al., 1991)

    Electric potential ofMichaelsen when

    creeping, mV

    Electric potential ofMichaelsen through

    a tube, mVPosition onbody Maximum Mean Maximum Mean

    Fore part 40 21 35 20Middle part 30 11 19 9Hind part 18 11 18 5

    their inherent contact angles according to the wettabilityanalysis of molecular fractal surfaces by Hazlett (1990).In this case, the inherent hydrophobicity of body surfacesof soil animals would be enhanced as the molecularfractal dimensions are increased.

    Solid surfaces on which contact angles of water arehigher have, in most cases, lower soil adhesion. As theycan enhance their inherent hydrophobic ability, themicroscopic geometrically non-smooth structure of thebody surfaces of soil animals is a surface morphologybene"cially reducing the soil adhesion to their bodysurfaces. The macroscopic non-smooth structure existing

  • body surface layer, the secretion layer and the soil layerin"ltrated with the secretion (Fig. 11). The sliding inter-face of the earthworm body surface against the soil oc-curred in the layer of the secretion near the soil layer,Fig. 9. Schematic diagram for the corrugated surface proxle ofsoil-borrowing animals (Tong et al., 1994d)

    on the body surfaces of soil animals can decrease theactual contact area of their body surface with soil andchange the contact state, so that the soil adhesion andresistance to motion can be reduced.

    Ren et al. (1990b) analysed the e!ects of the clawcon"guration of soil animals on the anti-adhesive charac-teristics. It was considered that the soil dug by the clawsof the soil animals cannot stick to, and easily slides along,the claw surfaces owing to the increase in the upwardcurvature of claw surfaces from the claw tip.

    4.2. Hydrophobicity

    It was found that the water "lm on the body surfaces ofsoil animals would rapidly contract and fall from theirbodies as they came out of wet soil. This phenomenonindicated that their body surfaces had a strong hydro-phobic nature. The organic substances, similar to resinand paint, including hydrophobic membranous proteinand non-polar radicals of phospholipid on the bodysurfaces contributed to their hydrophobic nature. Thehydrophobic ability of the body surfaces suggested thatthe attraction between the body surface substances andwater molecules was very weak. It can be considered thatthe body surface substances were low surface energymaterials. The inherent hydrophobic ability of the bodysurfaces of soil animals would be enhanced by theirgeometrically non-smooth or rough structure. The com-bination of the geometrically non-smooth structure andits hydrophobic nature is even better in preventing soilfrom sticking to the body surfaces of soil animals.

    LU-QUAN R2524.3. Microscopic electro-osmosis

    The action potential can be induced by stimulating thebody parts of soil animals. When soil animals are incontact with the soil, a microscopic electro-osmotic sys-tem would be formed in between the stimulated bodyparts and other parts nearby (Fig. 10). As a result, waterin the adjacent soil moves to the contact zones owing tothe action of the potential di!erence, the water "lm at thecontact interfaces becomes thicker, so that the soiladhesion to the body surfaces would be reduced throughFig. 10. Schematic diagram of the microscopic electro-osmoticsystem existing on the body surface of soil animals in contact with


    lubrication. Although the amplitude of the action poten-tial of soil animals is small, a microscopic electro-osmoticsystem can be formed because the distance between thepositive pole and the negative pole is very short. Thezone of negative polarity produced by stimulation fromthe contacting soil is on the same surface as the restingbody part near to the stimulating zone. For example, thepositive pole and the negative pole were on the segmentsnear to each other. The action potential of the bodysurfaces of soil animals has a dynamic distribution fea-ture. Therefore, this electro-osmosis was called non-smooth surface electro-osmosis (Cong et al., 1995a). Inaddition, the action potential would stimulate such soilanimals as earthworms to produce more secretions.

    4.4. ubrication

    Li et al. (1990) analysed the e!ect of the secretion ofearthworms upon its anti-adhesive characteristics. Themucoprotein as the main solute in the secretion had anopen and #at construction, giving a low tangential vis-cous drag. As the earthworm moved through the soil,some of the secreted liquid would in"ltrate into a layer ofthe contacting soil to form a three-layer system, i.e. the

    EN E A .since the liquid provided a weak shear layer. The lubri-cation phenomenon of the secretion of earthworms con-tributed to the reduction of soil adhesion to its body.

    4.5. Flexibility

    Ren et al. (1998) examined the vole fur for anti-adhes-ive and anti-frictional characteristics. Immediately afterremoving the skin from a freshly killed vole, soil adhesiontests were conducted in white clay with moisture contentsof 34 and 45% d.b., respectively. The soil adhesion to the

  • Fig. 11. Schematic diagram of a three-layer contact systembetween the soil and body surface of soil animals which secrete

    liquid substances

    vole skin was measured by comparison with that for

    test 22 biomimetic, embossed, bulldozing blades on thepseudovariable approximation design optimizingmethod. The biomimetic bulldozing blades were madefrom a plain carbon steel and were 400 mm long by200 mm wide. There were two arrangement patterns,a rectangle and a parallelogram (Fig. 13). The smallconvex domes embossed on the surface were 2}8 mm inheight, 16}32 mm in base diameter and 20}50 in number.Bulldozing tests were conducted in an indoor soil binusing a clay soil with an average moisture content of27)8% d.b. It was found that the bulldozing resistance ofthe embossed blades was reduced, on an average, by13)02% in comparison to a conventional (smooth) blade.The structural parameters of the optimum embossedsurface were convex domes, 7 mm in height, 25 mm in

    253SOIL ADHESION AND BIOMIMETICSsteel-45 [Fig. 12(a)] and found to be considerably less. Itwas observed that the vole fur was progressively separ-ated from the soil surface during the separating proced-ure of the adhesion tests, emphasizing the importance ofthe #exible feature of the fur. Friction tests of the vole furagainst soil were conducted at a sliding velocity of0)1 ms~1. The sliding direction of soil was along thegrowing direction of the vole fur on the body. It wasshown that the sliding resistance was reduced by about20% by comparison with steel-45 [Fig. 12(b)].

    5. Biomimetics of soil-engaging components

    5.1. Components with biomimetic non-smooth surfaces

    5.1.1. Bulldozing blades with biomimetic, embossedsurfaces

    Ren et al. (1995c) imitated the surface morphology ofthe &bulldozing blade of the dung beetle to design andFig. 12. Adhesion (a) and sliding resistance (b) of soil versus applcharacteristics of vole fur (Ren et al.base diameter and 45 in number, with a parallelogramarrangement. In the optimum case, bulldozing resistancewas reduced by 18)09% as compared to that of theconventional smooth blade.

    Qaisrani et al. (1993) and Qaisrani (1993) designedembossed, bulldozing blades with six arrangement pat-terns (Fig. 14). The small convex domes were made fromsteel-45 and ultra high molecular weight polyethylene(UHMWPE). The small convex domes, prepared in ad-vance, were glued on a conventional smooth steel-45plate of 250 mm in length and 130 mm in width. The testswere conducted on a soil cutting test table. The depthand the angle of cut were 15 mm and 353, respectively.The cutting speeds were 0)01, 0)02 and 0)06 m s~1, respec-tively. It was found that the bulldozing resistance of thesix biomimetic non-smooth bulldozing blades whoseconvex domes were made from UHMWPE was reducedto di!erent extents and the "fth blade (Fig. 14) had thelowest resistance, which was reduced by 27)50, 28)59 and34)0% at the cutting speeds of 0)01, 0)02 and 0)06 m s~1,ied normal stress, showing better anti-adhesive and anti-frictional, 1998); , steel-45; , vole fur

  • Fig. 13. Schematic diagrams showing two regular distributions of convex domes on the biomimetic embossed non-smooth bulldozingblades: (a) rectangle distribution; (b) parallelogram (Ren et al., 1995c)

    respectively. Of the six bulldozing blades whose convexdomes were made from steel-45, the "fth blade was theonly blade with a reduction in the bulldozing resistance,which was reduced by 1)3, 1)5 and 15)55% at the cuttingspeed of 0)01, 0)02 and 0)06 ms~1, respectively. The res-istance of the other blades was higher than that of theconventional blade.

    Li et al. (1995) conducted a statistical analysis of thecharacteristic parameters of the surface morphology ofthe &bulldozing blades of the dung beetle, including the

    proportion to the length of this segment, that is,N

    L"a#b (6)

    where a and b were parameters to be estimated, whichwere !0)2430 and 0)1692, respectively, for the dungbeetle tested. The base diameter of the convex domesranged from 0)033 to 0)749 mm. The size of the basediameter of the convex domes followed a Gaussian distri-bution on the basis of the s2 test and the mean and theroot-mean square deviation were 0)052 and 0)008 mm,

    LU-QUAN REN E A .254characteristic dimensions, and the number of the smallconvex domes, using a scanning electron microscopyphotograph. It was found that the position distributionof the small convex domes on the &bulldozing bladesurface followed a statistically uniform distributionbecause the number of the convex domes N


    a straight line segment along any direction was in directFig. 14. Schematic diagrams showing six regular distribution patterblades (Qaisrespectively.Based on the above statistical analysis, a biomimetic

    embossed bulldozing blade was designed by Ren et al.(1995b) (Fig. 15). The blade and the convex domes weremade from a plain low carbon steel (Q235A). The testswere run on a soil cutting test table using a clay soil witha moisture constant of 27% d.b. at the angle of cut ofns of convex domes on biomimetic embossed non-smooth bulldozingrani, 1993)

  • Fig. 15. Schematic diagram of a biomimetic embossed non-smooth bulldozing blade with a random distribution of small

    convex domes (Li et al., 1995; Ren et al., 1995b)

    31}463 (the angle of the blade with respect to the direc-tion of travel in the horizontal plane) and the cutting

    SOIL ADHESION Aspeeds of 13)33}58)82 mms~1. It was demonstrated thatthe biomimetic blade had a lower resistance to forwardtravel (Fig. 16 ).

    5.1.2. Bulldozing blades with biomimetic, corrugatedsurfaces

    Cong et al. (1996) imitated the wavy structure of bodysurfaces of some soil animals and designed a biomimetic,corrugated bulldozing blade (Fig. 17). The dimensions ofthe biomimetic blades prepared were 300 mm long by150 mm wide. Six biomimetic blades of di!erent waveparameters were prepared and cutting tests demon-strated that the soil adhesion to the conventional bladedid not occur on the corrugated blade. The bulldozingresistance of the biomimetic, corrugated blade was re-duced considerably in comparison with the conventionalblade. Chen et al. (1996) conducted "nite element analysisof the interaction between the soil and a corrugatedFig. 16. Forward resistance versus forward velocity, showinga reduced resistance for the biomimetic embossed non-smoothbulldozing blade with a random distribution of convex domescompared with a conventional blade (Ren et al., 1995b);

    , biomimetic blade; , conventional bladeFig. 17. Schematic diagram of biomimetic corrugated non-smooth bulldozing blade (Cong et al., 1996)

    bulldozing blade and predicted the horizontal and verti-cal components of the bulldozing resistance duringoperation.

    5.1.3. Bulldozing blades with biomimetic dimpledand scaly surfaces

    Cong et al. (1995b) designed biomimetic, non-smoothbulldozing blades imitating the dimpled form and thescaly form of surfaces of soil animals (Fig. 18). A com-parative test of the bulldozing resistance of the two bi-omimetic bulldozing blades in clay soil, wet sand anddried sand was conducted. It was found that the resist-ance against wet sand and dried sand was increasedmarkedly, while the resistance in a disturbed yellow claywas reduced considerably.

    5.1.4. Mouldboard ploughs with biomimetic embossedsurfaces

    Qaisrani et al. (1992) designed a biomimetic, embossedplough mouldboard. It was prepared by gluing smallconvex domes on the surface of a conventional mould-board (steel-35). The small convex domes had a basediameter of 20 mm and a height of 2 mm and were madefrom UHMWPE. Suminitrado et al. (1988) concludedthat the angles of the sliding direction of the soil on themouldboard surface with the horizontal surface was be-tween 55 and 653. Based on this phenomenon, an ar-

    255ND BIOMIMETICSrangement pattern of convex domes on the mouldboardwas designed (Fig. 19). The biomimetic plough was testedin an indoor soil bin using black clay soil. The ploughingresistance of this biomimetic non-smooth plough wasreduced by 26 and 34% at the forward speeds of 3)6 and5)0 kmh~1, respectively, as compared with a correspond-ing smooth mouldboard plough. The soil adhering to thebiomimetic plough surface was less than that to thesmooth plough.

    Li et al. (1996a) designed a biomimetic, embossedmouldboard plough with the same non-smooth structureas the biomimetic bulldozing blade surface shown inFig. 15, namely, the base diameter of small convex domeson the mouldboard surface was a Gaussian distributionand the position of the convex domes was a statistically

  • Fig. 18. Schematic diagram of the biomimetic bulldozing blades: (a) dimpled, and (b) scaly (Cong et al., 1995b)

    uniform distribution. The material from which the con- was also found that the Yangcheng mouldboard materialhad the best corrosion resistance. Li et al. (1996b) testedthe e!ect of the corrosion resistance of steel-35 on its

    LU-QUAN REN E A .256vex domes were made was a white iron alloy possessinga better anti-adhesive characteristic (Li, 1993). The whiteiron alloy was manufactured into a metal pencil. Theconvex domes on the mouldboard were preparedthrough weld accumulation on the white iron alloy ofa conventional mouldboard made from steel-35. It wasdemonstrated that the resistance of the biomimeticplough was reduced by 6)6}12)7% as compared to thecorresponding conventional mouldboard plough.

    5.2. Modi,cation of soil-engaging materials

    5.2.1. Iron and steelChen et al. (1995a) examined the e!ects of the chemical

    composition and microstructures on the water wettabil-ity, corrosion resistance and soil-scouring properties ofthree materials traditionally used for cast-iron mould-boards in China. The mouldboards made in Yangchengin Shanxi province of China had the best soil scouringproperty of the three traditional mouldboards. Its typicalcomposition featured higher carbon and phosphorus andlower manganese, silicon and sulphur content. A repre-sentative composition by weight of the mouldboardincluded carbon of 4)27%, phosphorus of 0)58%, manga-nese of 0)07%, silicon of 0)05% and sulphur of 0)032%. ItFig. 19. Schematic diagram of a biomimetic mouldboard plough(Qaisrani et al., 1992)anti-adhesive characteristic to soil. The microstructure ofsteel-35, hardened and tempered at a low temperature orhardened and tempered at a high temperature, had high-er corrosion resistance than that for the microstructurehardened and tempered at an average temperature, as didtheir anti-adhesive characteristic to soil. The soil ad-hesion to iron and steel was related to their individualrelative surface potential. A calomel electrode was takenas a standard electrode for comparison to determine therelative surface potentials of steel-35 of di!erent micro-structures using 0)1% NaCl solution as electrolyte(Li et al., 1997). The soil adhesion to steel-35 witha microstructure of a lower relative surface potential waslower (Fig. 20). This phenomenon seems to have a corre-sponding relation with the characteristic phenomenon ofthe action potential of soil animals.

    Li (1993) developed a white iron alloy and applied itfor preparing convex domes for the embossed surfaces.The white iron alloy had a lower relative surface poten-tial. In addition, the iron had a better abrasion resistance.The biomimetic, embossed plough whose convex domeson the mouldboard were made from the iron displayedFig. 20. Soil adhesion to steel-35 versus its surface relative poten-tial (Li et al., 1997)

  • Fig. 21. Ewects of size and content of Al2O3 particles on the polytparticles: (a) soil adhesion; (b) abrasive wear;

    a better anti-adhesion characteristic and lower ploughingresistance (Li et al., 1996a).

    5.2.2. Polymer composites and coatingsAlthough they had a better anti-adhesive character-

    istic, applications of polymeric materials to soil-engagingcomponents were limited because of their lower abrasivewear resistance against soil (Tong et al., 1994b, 1999;Chen et al., 1994). The composite material technique isa better method for increasing abrasive wear resistance ofpolymeric materials. The second phase particles or "bresexisting in composite materials can make their surfacespossess a feature similar to the geometrically non-smoothtopography of the body surfaces of soil animals.

    Tong et al. (1990b) and Lu et al. (1996) prepared an

    SOIL ADHESION Aalumina particle reinforced Te#on composite material.When the alumina particle content was less than 20% byweight, its abrasive wear resistance was increased con-siderably and the soil adhesion was a!ected a little(Fig. 21). Liu et al. (1998a, 1998b) tested the e!ects ofquartz particles and glass beads as the reinforced mater-ials upon soil adhesion and abrasive wear of UHMWPE.The anti-adhesive characteristic of the UHMWPEmatrix composite was little a!ected, as long as the vol-ume content of the reinforced materials in the compositewas not more than 9% by volume.

    Jia et al. (1993) analysed the water wettability of com-posite materials reinforced with particles. Based on theassumption that (a) the particles in composite materialswere spherical and had the same diameter which wassmaller than the water droplet during tests of contactangles of water on the surfaces of composites, (b) theparticles were uniformly distributed in the matrix of thecomposite materials, and (c) the outer surfaces of par-ticles were circular with variable diameters, they deriveda theoretical equation for the particle reinforcedetrayuoroethylene (PTFE) matrix composite reinforced with Al2O3, 120 mesh; , 280 mesh (Lu et al., 1996)

    composite materials as follows:

    cos hc"cos h



  • Fig. 23. Photographs of two biomimetic bulldozing blades withelectro-osmosis: (a) embossed; and (b) corrugated (Ren et al.,

    forward speed was 0)95}1)88 m s~1. The above resultsshowed that the combination of hydrophobic materialsand geometrically non-smooth surfaces was a bettermethod for reducing adhesion and interfacial friction ofsoil-engaging components.

    5.3. Biomimetic non-smooth electro-osmosis

    for the zones of positive and negative polarity of8}12)5%. The traction force and the driving e$ciency of

    LU-QUAN REN E A .2585.3.1. he basic structure of biomimeticelectro-osmotic surfaces

    Based on the characteristics of the surface action po-tential of soil animals, a combined measuring dome wasdesigned (Fig. 22) by Cong et al. (1995a) with a positivepole in the centre and a negative pole at the periphery.This combined disc was used for testing, in principle, thee!ect of the electro-osmosis upon soil adhesion. The soilused had a moisture content of 23)5% d.b. The voltagefor electro-osmotic tests was 12 V. The applied normalstress was 26)78 kPa. It was found that there existed anarea ratio for the zones of positive and negative polaritiesthat resulted in minimum soil adhesion. In other words,the ratio of the area of positive pole to negative pole wasan important factor a!ecting soil adhesion. This electro-osmotic system in which the positive pole and negativepole were on the same surface and the positive pole wason a convex dome was called non-smooth surfaceelectro-osmosis or biomimetic electro-osmosis.

    5.3.2. Bulldozing blades with biomimeticelectro-osmosis

    According to the principal test results, Cong et al.(1995a), Ren et al. (1995a) designed two biomimetic bull-dozing blades with non-smooth surface electro-osmoticstructures. One was the convex dome positive pole[Fig. 23(a)] and another was the corrugated form of thepositive pole [Fig. 23(b)]. Bulldozing tests were conduc-ted on a soil cutting test table. The forward speed wasfrom 43 to 172 mms~1, the depth cut was 50 mm, theangle of cut was 603, the voltage for electro-osmosis was12 V and the ratio of area of the positive pole to theFig. 22. Schematic diagram of a biomimetic bulldozing blade withan embossed non-smooth surface structure for elecro-osmosis

    (Cong et al., 1995a)1995a)

    negative pole was 1 : 7)3. It was found that when soilstuck on the surface of the conventional blade and thenon-smooth surface without electro-osmosis, only a littleor no soil stuck to the surface of biomimetic, electro-osmotic plate under identical conditions. Biomimeticelectro-osmosis can further reduce bulldozing resistanceby 9}12% and 15}32% for the embossed and for thecorrugated non-smooth surfaces, respectively, as com-pared with the corresponding non-smooth surface with-out electro-osmosis.

    5.3.3. =heel lugs with biomimetic electro-osmosisThe electro-osmotic principle for non-smooth surfaces

    was used by Chen et al. (1995b) for the lugs of paddy-"eldwheels. They designed varied convex forms of biomimeticlugs (Fig. 24). The tests were conducted in an indoor soilbin using supersaturated black clay soil similar to thatfound in paddy "elds. The working depth of lugs was150 mm. Soil blockage due to adhesion occurred on theconventional lugs without electro-osmosis and theblocked soil was not self-cleaning, while only a little soiladhered to the lugs with biomimetic electro-osmosis andthe lugs were self-cleaning. When a voltage of 24 V wasused for electro-osmosis and convex domes with a basediameter of 24 mm were taken as positive poles, the soilshedding property of lugs was the best at an area ratioFig. 24. Schematic diagrams of biomimetic electro-osmotic sur-face structures used for the lugs of paddy-xeld wheels (Chen et al.,


  • 259SOIL ADHESION AND BIOMIMETICSFig. 25. Ewects of biomimetic electro-osmotic lugs on a paddy-xeld, with electro-osmosis;

    paddy-"eld wheels were increased owing to biomimeticelectro-osmosis (Fig. 25).

    5.4. Flexible components for removing soil

    5.4.1. oading bucket with a -exible liningSun et al. (1992, 1996a, 1996b) designed a #exible steel

    mesh based on the #exibility features of soil-burrowinganimals. The steel mesh was made from linked steel ringsand slotted steel rings [Fig. 26]. The steel mesh was madeinto a lining suitable for such soil-engaging componentsas digger buckets and wheel buckets. The position of thelining could be changed with the position of the bucket asshown in Fig. 27. When the bucket with a steel meshlining was "lled with soil, the lining was pressed close tothe inside of the bucket. At the beginning of unloading,Fig. 26. Schematic diagram of structure of a yexible steel meshlining (Sun et al., 1992)

    both soil and the lining slid out from the bucket simulta-neously, and then the soil was separated from the lininggradually by gravity and, "nally, the lining was hungfrom the bucket. Only the smaller size of soil particlescould enter between the lining and the inside of thewheel: (a) tractive force, (b) tractive ezciency (Chen et al., 1995b);, without electro-osmosis

    bucket through the gap in the lining, since the size of thegaps in the lining was very small. In order to protect soilfrom blocking between the lining and the inside of thebucket, several holes were drilled through the bottom ofthe bucket shell to release the trapped soil.

    5.4.2. Self-unloading box of dump truck with-exible chains

    Wang et al. (1997) tested the e!ect of three #exiblelinings, steel mesh, steel chain and nylon-knittednet, on a model of a self-unloading box. The tests wereconducted using yellow clay with a moisture content of24)5% d.b. After 20 successively loading}unloadingoperations, the soil adhering to the box without #exiblelining was 14% by volume of the total holding capacityof the box. Heavy soil adhesion took place after 10Fig. 27. The position of the yexible lining with respect to thebucket (Sun et al., 1996b)

    loading}unloading operations for the nylon-knitted netand, afterwards, it lost the #exible function and soil-shedding action. The nylon-knitted net was found to bebroken easily. No soil was built up on the steel meshlining and the steel chain lining after 20 loading}unload-ing operations. The steel mesh lining and the steel chainlining would increase the deadweight of the self-unload-ing box, which is their disadvantage.

  • 6. Summary

    The phenomenon of soil adhesion existing between soiland the soil-engaging surfaces of varied terrain machinesincreases their working resistance and energy consump-tion and decreases their quality of work. The adhesionresults from the capillary pressure and the viscous resis-tance of the water "lm at the interface for wet soil, wasmainly due to the molecular attraction and the negativeair pressure for dry soil. The factors a!ecting soiladhesion include the nature and properties of the soil, theproperties of the soil-engaging component surfaces andthe experimental conditions. The conventional methodsfor reducing soil adhesion include the design of surfaceshapes, selection of surface materials for soil-engagingcomponents and application of electro-osmosis, mag-netic "elds, vibration and lubrication.

    Soil-burrowing animals have signi"cant anti-adhesivecharacteristics through evolution over millions of years.The reasons why the body surfaces of soil-burrowinganimals do not stick to soil includes varieties of geomet-rically non-smooth morphologies, chemical compositionand hydrophobicity, microscopic electro-osmotic sys-tems formed by their bioelectricity and #exibility of theirbody surfaces and the lubrication action of the liquidsubstances on their body surfaces as well. Various bio-mimetic methods were developed on the basis of theanti-adhesive principles of soil-burrowing animals. Thesebiomimetic methods were the development of non-smooth surfaces for soil-engaging surfaces, electro-osmo-tic systems, and #exible parts for removing soil accretionon the soil-engaging components.


    This project was supported by the National NaturalScience Foundation of China (Grant No. 59835200) andDoctoral Foundation of China (Grant No. 97018514).


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    Notation1. Introduction2. Adhesion of soil to solid surfacesFigure 1Table 1Figure 2Figure 3Figure 4Figure 5

    3. Characteristics of soil-burrowing animalsFigure 6Figure 7Table 2

    4. Principles of anti-adhesive characteristics of soil animalsTable 3Figure 8Table 4Figure 9Figure 10Figure 11

    5. Biomimetics of soil-engaging componentsFigure 12Figure 13Figure 14Figure 15Figure 16Figure 17Figure 18Figure 19Figure 20Figure 21Figure 22Figure 23Figure 24Figure 25Figure 26Figure 27

    6. SummaryAcknowledgementsReferences


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