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

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<ul><li><p>ai</p><p>and energy consumption of these machines, but also decreases the quality of work. Characteristics of soil adhesion to</p><p>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.</p><p>( 2001 Silsoe Research Institute</p><p>1. Introduction</p><p>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 &amp; 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</p><p>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.</p><p>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.</p><p>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</p><p>REVIEW</p><p>Soil Adhesion and Biomimetics of S</p><p>Lu-Quan Ren; Jin Tong; Jia</p><p>Research Institute for Agricultural Machinery Engineering, Jilin Univere-mail of corresponding</p><p>(Received 30 August 1999; accepted in revised for</p><p>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</p><p>0021-8634/01/070239#25 $35.00/0 23.idealibrary.com on</p><p>PAPER</p><p>oil-engaging Components: a Review</p><p>n-Qiao Li; Bing-Cong Chen</p><p>sity (Nanling Campus), Changchun 130025, Peoples Republic of China;uthor: jtong@jlu.edu.cn</p><p>m 15 March 2001; published online 6 June 2001)</p><p>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</p><p>9 ( 2001 Silsoe Research Institute</p></li><li><p>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 &amp;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.</p><p>2. Adhesion of soil to solid surfaces</p><p>2.1. Morphological features of soil at the contactinterfaces and contact models</p><p>2.1.1. Surface morphologies of soil at the contact interfacesThe soil surface morphologies formed at the contact</p><p>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</p><p>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.</p><p>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 )].</p><p>2.1.2. Contact models of soil with solid surfacesOn the basis of the water "lm states and the surface</p><p>morphology at the contact interface, there were twoNota</p><p>a constantb constantD fractal dimensionF tensile force, Nh1</p><p>distance between two solid surfaces, mh2</p><p>distance between two solid surfaces, mJ some property of the interlayer water</p><p>J0</p><p>property J of bulk water length of segment, m</p><p>M(r(R) cumulative mass of soil particles, kgM</p><p>Tmass of the total soil particles, kg</p><p>mm</p><p>mass of soil, kgm</p><p>wmass of water, kg</p><p>NL</p><p>number of embossed convex domesP capillary pressure, PaR characteristic size of particle, m</p><p>Rd</p><p>disc radius, mR</p><p>Lthe size of the largest soil particle, m</p><p>RT</p><p>radius of the theoretical capillary tube, mr soil particle size, m</p><p>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</p><p>ra</p><p>ratio of areast time, s&lt; volume content of reinforcement in</p><p>a composite materiala tilt angle, degb constant</p><p>cLV</p><p>surface tension of water, Nm~1g viscosity of liquid, Pa s</p><p>hA</p><p>contact angle of advancing water, deghR</p><p>contact angle of receding water, degh0</p><p>inherent contact angle of water on sur-face, deg</p><p>h1</p><p>contact angle of water on matrix mater-ial, deg</p><p>h2</p><p>contact angle of water on reinforcingmaterial, deg</p><p>ha</p><p>apparent contact angle of water onrough surface, deg</p><p>hc</p><p>contact angle of water on compositesurface, deg</p><p>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</p></li><li><p>oFig. 1. Photographs by scanning electron microscopy of the morphc) contacting with a disc of Teyon, and (d) contacting with a rubbe</p><p>SOIL ADHESION A27)1% d. b. and (d) supersaturated b</p><p>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.</p><p>2.2. Explanations of soil adhesion</p><p>2.2.1. Moisture tension of the interfacial water ,lmFountaine (1954) demonstrated that water "lm plays</p><p>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:</p><p>P"2cLV</p><p>/RT</p><p>(1)</p><p>where cLV</p><p>is the surface tension of water, RT</p><p>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</p><p>241ND BIOMIMETICSlack soil (Tong et al., 1993, 1994c)</p><p>Eqn (1), the adhesion is very strong when the water "lm isvery thin and vice versa.</p><p>2.2.2. Molecular attraction and negative air pressureThere exists molecular attraction between the solid</p><p>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.</p><p>2.2.3. Capillary attraction from the interfacialwater ,lm</p><p>Akiyama and Yokoi (1972a, 1972b) represented soilparticles in the geometrically simpli"ed form of spheres</p></li><li><p>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.</p><p>2.2.4. </p></li><li><p>lele</p><p>ag</p><p>NTabAdvancing contact angles hA and receding contact ang</p><p>Solid materialAdvancing cont</p><p>(hA</p><p>), de</p><p>Polytetra#uoroethylene (Te#on) 109Polystyrene 91Polyethylene 96Polypropylene 108Para$n wax 110Polycarbonate 84Human skin 90Gold 66Platinum 40Plain carbon steel 73Enamel 29</p><p>of a soil is a fractal, then the log}log plot of M(r(R) andR/R</p><p>Lis a straight line. The fractal dimension D can be</p><p>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.</p><p>2.3.2. E+ects of properties of soil-engaging componentsurfaces</p><p>The atomic or molecular structure of surfaces of a solidor a liquid is di!erent to that of their bulk materials</p><p>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</p><p>ct angle Receding contactangle (h</p><p>R), deg Reference</p><p>106 Wu (1982)84 Wu (1982)62 Wu (1982)</p><p>Adamson (1976)Adamson (1976)</p><p>68 Wu (1982)Adamson (1976)Adamson (1976)Adamson (1976)</p><p>0 Tong et al. (1994c)0 Tong et al. (1994c)</p><p>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</p><p>Aand</p><p>receding contact angles hR</p><p>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.</p><p>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</p><p>243D BIOMIMETICSquenching and average temperature tempering t...</p></li></ul>