1. Introduction
In most soil-engaging parts in tillage and planting tools, the soil slides tangentially along the tool surface. The sliding resistance along the soil–tool interface is determined by three forces that interact: (i) structural resistance to soil particle displacement, (ii) frictional resistance to transfer between individual soil particles, and (iii) resistance produced by soil–tool adhesion [
1]. Soil adhesion to the tool surface can cause a change in surface shape and substantially increase the sliding resistance.
Soil adhesion is the force of attraction between soil particles and the surface contacting the soil; this attraction often occurs due to the surface tension and viscosity of the interfacial water film (a few molecules thick) forming on the tool surface [
2]. Soil–tool adhesion can have various implications for agricultural operations, including lowering germination rates [
3], increasing energy consumption per unit soil operation [
4], and restricting the use of no-tillage planting in sticky soils [
5]. Another consequence of soil adhesion phenomena is a considerable reduction in the work efficiency of loading and excavation equipment [
6].
According to Coulomb’s theory, the sliding resistance along the soil–tool interface could be described as a function of the normal load applied to the soil–tool interface, the soil–tool contact area, soil–tool adhesion, and soil–tool friction as per Equation (1) [
7],
Here,
is the sliding resistance in the tangential direction, (N),
is the contact area, (cm
2),
is the coefficient of adhesion, (N/cm
2),
φ is the soil-metal friction angle (degrees), and
is the normal load to the contact interface (N).
Fountaine (1954) [
8] initially proposed Equation (1) based on laboratory experiments concerned with the mechanism of soil-metal adhesion, which was later applied to non-metallic materials such ultra-high molecular weight polyethylene [
7] and fiber-glass [
9]. As a result, Equation (1) could be used to characterize soil-tool sliding resistance regardless of the material used.
In other words, soil–tool sliding resistance includes two terms, namely, friction and adhesion. However, Srivastava et al. [
10] pointed out that it is difficult to distinguish between friction and adhesion. As a result, an apparent coefficient of friction is commonly employed to account for both friction and adhesion effects.
According to Soni and Salokhe [
11], many factors impact soil–tool adhesion, including soil texture, soil moisture content, tool material, tool geometry, interfacial conditions, and soil Atterberg constants, particularly the sticky point. The sticky point is the soil moisture content where the soil begins to cling to a foreign item. More precisely, it is the soil paste’s moisture content at which the soil particles begin to attach to a polished nickel surface under a shearing speed of 50 mm s
−1 [
12].
To date, a variety of techniques have been used to minimize soil–tool adhesion, including changing the composition of the material used to fabricate soil-engaging parts [
9], altering the geometric shape of implements [
13], generating mechanical and ultrasonic vibrations [
14], increasing the adaptability of the soil-engaging components [
15], using polymeric materials to coat soil-engaging parts [
7,
16], and using a bionic electro-osmosis method [
17].
In light of the promising results of surface shape modification in lowering soil adherence to primary and secondary tillage implements under certain soil conditions [
18,
19], it was a natural step to expand experiments to incorporate a broader range of soil conditions, such as those in paddy fields. Many mechanized agricultural operations are carried out during rice transplanting seasons in Hubei Province, China, such as field flattening, fertilization, and direct seeding. During these operations, a large amount of soil clings to soil-engaging components, limiting their efficiency and effectiveness (
Figure 1). In this experiment, an L27 (3
3) Taguchi design was employed; the Taguchi design is a frontier in data mining techniques that has gained popularity in engineering applications [
20]. The Taguchi technique uses signal-to-noise ratio analysis as a measurable statistic “quantitative analysis tool” to find optimal levels of control parameters that lead to an optimized design [
21]. The current research aimed to explore the effects of specific geometric dimensions (disc coverage ratio, dome height-to-diameter ratio, and dome base diameter) on drag force under paddy fields conditions. Furthermore, the drag force of the optimized domed disc was compared to that of the flat disc under different soil conditions.
4. Discussion
Statistics revealed that the dome height-to-diameter ratio and disc coverage ratio were crucial parameters that influence the drag force of domed surfaces, while the dome base diameter had limited influence. The dome height-to-diameter ratio was arguably the most dominant parameter, with a percentage contribution of 58.5%. The dominance of the dome height-to-diameter ratio was mainly due to the close linkage between the capacity of domes to break the continuity of the interfacial water film and the extent of dome convexity [
28,
29]. Moreover, the dome height-to-diameter ratio affected the extent of disc sinkage into soil paste, hence, soil resistance on the disc’s front side. The disc coverage ratio also showed a significant contribution (29.4%) to the disc drag force. The results showed that the higher the disc coverage ratio, the more disc drag force. At a coverage ratio greater than 60%, the increase in drag force could be attributed to a blockage of soil movement between adjacent domes, causing the soil to accumulate in front of the disc along the disc stroke.
Results revealed that the lowest drag force values were associated with the HDR range of 15% to 30%, while domed discs with a dome height-to-diameter ratio of 37.5% had increased drag force of up to 20% (Disc 9) compared to a flat disc. In contrast, Soni and Salokhe [
23] reported that domed surfaces with a dome height-to-diameter ratio of 50% or less had decreased drag force. This contrast in results may be related to the different soil textures used in both experiments, since Soni and Salokhe [
23] used heavy clay soil with 62% clay, whereas the current study used silty loam and sandy clay loam soils with 15% and 21% clay, respectively. According to Srivastava et al. [
10], the fine soil particles (clay and fine silt) have a higher capacity for water-holding due to their large surface area; thus, heavy clay soil is stickier than silty loam and sandy clay loam soils.
The optimized domed disc (Disc 14) successfully reduced drag force compared to the flat disc under varying soil moisture contents ranging from 23% to 37%. The maximum recorded drag force reduction was 25% in the silty loam soil with 30% moisture content. The reduced drag force of Disc 14 could be attributed to the ability of domes to push the soil away from concave areas between adjacent domes while dragging, restricting interfacial water film continuity. Another reason for the drag force reduction by the optimized domed disc was air retention in the concave areas between adjacent domes, which provided a gas isolation layer, reducing soil-disc adhesion. Furthermore, the tangential movement of the optimized domed disc caused disturbances in the surrounding soil layer, resulting in a decrease in the soil–disc friction coefficient [
30].
The drag force test in silty loam soil (S1) revealed that raising the soil moisture content below the liquid limit increased drag force. This increase in drag force associated with the increase in soil moisture content was probably due to the formation of a thin interfacial water film between the disc and the soil, increasing soil–disc adhesion. The soil–disc adhesion, therefore, acted as an extra load, resulting in increased drag force. On the other hand, the expected increase in initial disc subsidence due to rising soil moisture content could increase soil resistance on the disc’s front side, hence drag force. In comparison, the soil–disc sliding resistance in sandy clay loam soil (S2) followed a distinct pattern. The soil–disc sliding resistance was initially low at the lowest moisture content (23%); as the moisture content increased to 30%, the soil–disc sliding resistance increased. Finally, as the moisture content increased even further, the soil–disc sliding resistance decreased. The marked difference in sensitivity to changes in soil moisture content between S1 and S2 was probably attributable to differences in fine soil particle content (clay and fine silt). In contrast to coarse soil particles, fine soil particles have a high water-holding capacity due to their large surface area and chemical interactions. Due to the lower fine soil particle content of S2, the sliding resistance was reduced as the moisture content reached the liquid limit due to the lubricating effect generated by the interfacial free water layer [
8].
5. Conclusions
The micro-convex structure of the dung beetle head was used as a biomimetic prototype to create a number of domed discs in order to examine the influence of specific geometrical parameters on sliding resistance reduction using an L27 (33) Taguchi orthogonal array. Variance analysis (ANOVA) revealed that the dome height-to-diameter ratio was the most dominant parameter, followed by the disc coverage ratio. In contrast, the dome base diameter had a limited influence on drag force. Using the signal-to-noise ratio analysis per the lower-is-better strategy, the optimal disc for minimizing drag force was determined to be Disc 14, with a disc coverage ratio of 60%, a dome height of 5 mm, and a dome base diameter of 20 mm.
The sliding resistance of the optimized domed disc (Disc 14) and that of the flat disc were compared under certain soil conditions (soil textures of silty loam and sandy clay loam; soil moisture contents of 23%, 30%, and 37%) to investigate the biomimetic effect of the optimized domed disc. The optimized domed disc produced less sliding resistance than the flat disc in all treatments, by around 9% to 25%, depending on the soil conditions. The results obtained in this experimental study can be used to support the manufacture of paddy soil–engaging components such as flattening tools and fertilizer furrow openers.