Investigating Soil Adhesion and Antiadhesion Performance of Nonsmooth Subsoiler Surfaces

Abstract

Soil adhesion is one of the main factors affecting the working resistance and energy consumption of agricultural machinery. Many scholars have been doing much research on the method of soil antiadhesion, but the conditions for the occurrence of adhesion need to be further discussed. By measuring and comparing the repose angle and external friction angle of the soil, this paper discussed the influence of soil texture and soil water content on soil adhesion. It was found that when the soil moisture content was less than 20%, the cohesive soil showed weak adhesion. When the soil moisture content was more than 20% and lower than the soil liquid limit, the cohesive soil shows strong adhesion. When the soil shows strong adhesion, the nonsmooth convex structure on the surface of the subsoiler has a good desorption effect. Notably, in clay loam with a 30% moisture content, the average tillage resistance of a subsoiler with convex structures was 15.13% lower compared to a smooth-surfaced subsoiler.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Tillage is one of the necessary methods for achieving stable and increased crop yield. However, traditional deep tillage, rotary tillage, and other large disturbance tillage methods can reduce soil vegetation coverage, leading to soil desertification and thinning of the arable layer due to wind and water erosion [1]. As a form of conservation tillage, subsoiling can effectively break the plow pan layer, improve the structure of the plow layer, and maximize straw coverage on the soil surface while meeting the basic growth needs of crops. This helps to reduce soil erosion, protect the ecological environment of the farmland, and achieve ecological and economic benefits [2–5].

The purpose of the subsoiling is to break up the tillage layer, which means that the depth of tillage must be at least as deep as the tillage layer. According to the formation mode of the tillage layer and field measurement, subsoiling operation in most areas requires an effective tillage depth of 30–40 cm. The primary factors affecting the working resistance of tillage components are deep tillage during subsoiling and soil adhesion of soil-engaging components. Soil adhesion not only increases tillage resistance and energy consumption but also affects the operational efficiency and work quality. The high resistance to subsoiling and the challenges of matching small and medium tractors on the market are limiting factors for the widespread adoption of agricultural mechanization [2].

In recent years, numerous scholars have conducted extensive research and proposed various theories of viscosity and desorption in agricultural machinery. The application of material science [6], vibration drag reduction [7–9], soil improvement [10–12], and electroosmosis drag reduction [13] has good effects in reducing adhesion and desorption. In addition, the development of bionics provides a new idea for reducing adhesion and desorption in tillage components. The theory of profiling design and nonsmooth surface drag reduction has rapidly advanced and gained widespread recognition [14–17]. Nonsmooth surface structures have also demonstrated effective antiadhesion properties on the soil-engaging components of agricultural machinery, such as convex, concave, rib, and other structures [18–20].

Although the nonsmooth surface exhibits good desorption properties in soil, its complex structure, challenging processing, and high manufacturing cost are notable drawbacks. In addition, the soil environment is complex and diverse, making the nonsmooth surface susceptible to wear [21]. Consequently, it is difficult to play the maximum value of nonsmooth surface desorption drag reduction when working in nonadhesive soil. Some scholars have investigated the physical and chemical properties of soil in terms of soil texture, soil moisture content, and other factors to better understand soil-metal interactions. When the soil texture or moisture content changes, the adhesion between the soil and the metal surface will also change greatly. Nevertheless, the quantitative relationship among soil adhesion, soil texture, and water content remains unexplored. Therefore, further research is needed to elucidate the impact of soil texture and moisture content on soil adhesion under real working conditions.

In order to investigate the effect of different soil textures and moisture content on the adhesion between soil and subsoiler surface and the desorption ability of the structure on the nonsmooth surface, this research analyzes the mechanism of soil adhesion according to the measured resistance, soil properties, and adhesion phenomenon of standard subsoiler. Then, the soil repose angle and external friction angle were used to characterize the internal and external friction characteristics of the soil. Soil samples with different textures and moisture contents were selected and prepared. Through measurements and comparative analysis of these angles, the study investigates the influence of soil’s frictional characteristics on the adhesion between soil and subsoiler surfaces. Moreover, two distinct subsoilers with nonsmooth surface, along with a standard subsoilers with a smooth surface, were prepared, and soil adhesion and nonsmooth surface desorption were verified in soils with different moisture contents of the same texture. The findings of this study lay a theoretical foundation for understanding soil-metal adhesion under practical working conditions and offer insights for the optimal selection of subsoilers in subsoiling operations.

2. Materials and Methods

2.1. Field Experiment

To explore the mechanism of soil adhesion and the effect of adhesion on subsoiling resistance, the subsoiler tillage resistance was measured and the adhesion phenomenon was observed at different times and places.

2.1.1. Test System

The tillage resistance of the subsoiler was assessed using the field tillage resistance test system. As shown in Figure 1, the test system is a field force measuring system (a), mainly composed of a top link sensor (b), a left suspension sensor (c), a right suspension sensor (d), a data acquisition instrument (e), and a force measurement software interface (f). In the force measurement system, three sensors are utilized to quantify forces in both the horizontal and vertical axes: the left and right suspension sensors, along with the upper pull rod sensor. The software component of this system calculates and provides direct readings of the horizontal and vertical forces exerted on the agricultural machinery during operation. The test system has a maximum range of 45 kN and a minimum measurement accuracy of 0.045 kN.

[figure(s) omitted; refer to PDF]

2.1.2. Subsoiler Model Selection and Test Field Site Selection

As shown in Figure 2(a), a standard shovel with a smooth surface was chosen for the subsoiler experiment. The selection of field measurement sites comprised the following four distinct locations: Shijiazhuang City in Hebei Province, Gongzhuling City in Jilin Province, Siping City in Jilin Province, and Changchun City also in Jilin Province. These sites were strategically selected based on their varying working conditions. As shown in Figures 2(a)–2(d), 150 m × 10 m operational zone was demarcated for testing purposes, ensuring uniform conditions. The longitudinal axis of 150 m represented the working direction, with a plowing depth set at 30 cm. The operating speed was maintained at 3.6 km/h, while the horizontal subsoiling row spacing was set at 3 m to prevent any interference between adjacent rows during the subsoiling process. Each measurement was repeated thrice, and the average was subsequently calculated.

[figure(s) omitted; refer to PDF]

2.1.3. Measurement of the Soil Texture and Moisture Content at Site Locations

The soil texture of the test field was classified by sieving with an analytical sieve as depicted in Figure 3(a) to remove humus, rhizomes, and other organic materials. Each type of soil was analyzed for various soil particle size contents using a laser particle size analyzer (BT-9300ST), as shown in Figure 3(b) [22]. Finally, the soils were texturally classified according to the International Standard for Soil Texture Classification (ISSC).

[figure(s) omitted; refer to PDF]

As shown in Figure 3(c), the moisture content of the soil was measured by taking multiple soil samples, each in triplicate. In Figure 3(d), we placed the soil samples in a 102°C vacuum drying oven for 4 hours and then removed, naturally cooled, and measured the weight after it dried. We repeated the above steps until the error of two consecutive weight measurements is within 1 g. We calculated the dry basis moisture content of each soil sample and found the average value of the dry basis moisture content of three soil samples.

2.2. Measurement of the Soil Repose Angle and External Friction Angle

2.2.1. Soil and Metal Sample Preparation

In this article, one kind of forest humus soil and four types of representative field soil were selected, which were named humus soil from Heilongjiang Province, loess soil from Henan Province, black soil from Jilin Province, loess soil from Sichuan Province, and laterite soil from Yunnan Province according to the color and site selection of the soil, as shown in Figure 4.

[figure(s) omitted; refer to PDF]

In the preparation of soil samples, first of all, take a number of various types of soil and place them in a vacuum drying oven and dry at a temperature of 108 ± 2°C for 24 h. Take the dried soil, using the analytical sieve to remove rhizomes, large pieces of gravel, etc., with an electronic balance of the five types of soil according to the 500 g of each weighing 6 copies, a total of 30 copies. Each type of soil according to the dry basis of the moisture content of 0%, 10%, 20%, 30%, 40%, and 50%, and in turn, add water of 0 g, 50 g, 100 g, 150 g, 200 g, and 250 g and then seal and leave to stand for 48 h, to allow the soil to fully absorb water.

Soil firmness was assessed using the static contact test method, which is widely employed internationally. The SC-900 soil firmness instrument was utilized in this study to determine the soil penetration resistance of the probe, from which soil firmness parameters were derived. Following multiple measurements, the soil firmness values ranged from 1000 to 1500 kPa. In the preparation of soil blocks, first, 20 × 20 × 20 mm molds were filled with various types of soils and compacted under a pressure of 1200 kPa for 1 min to prepare spare soil blocks with length and width of 20 × 20 mm and thickness of not less than 10 mm. Three portions of soil blocks were prepared for each type of soil with different textures and moisture content.

In the preparation of the metal samples, first, the national standard material of subsoiler Q275 [22] was chosen, and the metal samples with the length, width, and thickness of 100 × 100 × 5 mm were prepared, and the surface was smoothed with 1500# sandpaper to remove the rust and then simulated the working environment of the subsoiler so that the metal samples were fully friction with the soil to form the friction surface of the interaction between the soil and the metal.

2.2.2. Soil Texture Analysis Test

The soil texture was classified using a method similar to the one described above taking various soils of each type. As shown in Figure 3(a), soil samples were sieved with an analytical sieve and selected to remove humus rhizomes, etc. Each type of soil was analyzed for various soil particle size contents using a Laser particle size analyzer (BT-9300ST) as depicted in Figure 3(b). Finally, the soils were texturally classified based on the ISSC guidelines.

2.2.3. Measurement of the Soil Repose Angle

The internal frictional properties of soil are inherently reflected in the cohesion between its particles. The soil repose angle defined as the angle between the surface of soil accumulation and the horizontal plane during natural deposition serves as an outward indicator of these internal friction characteristics. In this paper, the soil repose angle is measured to characterize the internal friction properties of the soil.

The soil repose angle is quantified through a soil accumulation test. As shown in Figure 5, this type of cylinder was chosen to measure the soil repose angle because some of the soil samples were sticky. Taken the prepared soil, we mixed it evenly, made the soil in a loose state, placed it in a bottomless cylinder, and then took away the cylinder vertically. On the carrier platform, the soil was piled up at different angles in the natural state due to the combined effect of self-weight and internal friction. After the soil base was stabilized, the angle of inclination of the soil base was measured, and the abovementioned steps were repeated three times to take the average value. Taken the soil with different textures and different moisture content, we repeated the abovementioned steps and measured the soil repose angle with different textures when the moisture content was 0%, 10%, 20%, 30%, 40%, and 50%, respectively. Since the soil with 50% moisture content has exceeded the soil liquid limit, rendering the soil highly fluid, thus the measured inclination is provided for comparative purposes only.

[figure(s) omitted; refer to PDF]

2.2.4. Measurement of the Soil External Friction Angle

The friction between soil and nonsoil surfaces manifests the soil’s external friction characteristics. The soil inclined plane test can measure the friction angle between soil and metal surface, that is, the soil external friction angle [23]. As shown in Figure 6, a metal template is mounted on an inclined test bench. The test face of the metal template was upward. We took all kinds of soil blocks prepared on-site and laid them flat on one side of the metal template. The inclination of the bench is adjusted to cause the template to rotate gradually around one edge until the soil blocks slide downward at a consistent velocity and recorded the inclined angle of the inclined plane. We wiped the test face of the metal template to dry and repeated the abovementioned steps three times to take the average value. We took soil blocks with different moisture contents and repeated the abovementioned steps to measure the external friction angle of various soils at moisture contents of 0%, 10%, 20%, 30%, and 40%, respectively. Because the soil with 50% moisture content has exceeded the soil liquid limit, the soil fluidity is strong and the soil block cannot be shaped, so the external friction angle of the soil with 50% moisture content was not measured.

[figure(s) omitted; refer to PDF]

2.3. Test of Soil Bin

To verify the antiadhesion of the nonsmooth surface of the subsoiler, the subsoilers with convex structure and concave structure on the surface were manufactured according to the existing theory of nonsmooth surface structure design theory [24–26], as shown in Figure 7.

[figure(s) omitted; refer to PDF]

Because the soil moisture content in the field is difficult to control, it is very difficult to verify the effect of subsoiler detachment on nonsmooth surfaces by the control variable method. In this paper, indoor soil bin equipment was used to design the soil bin test, as shown in Figure 8(a), and the force measurement system used was similar to that in Figure 1. As shown in Figure 8(b), flood irrigation was used to saturate the soil moisture content, after which the soil was left to stand, settle naturally, and evaporate, during which the soil moisture content was continuously measured. When the soil moisture content reached 40%, 30%, 20%, and 10%, the test vehicle was loaded sequentially with a standard subsoiler and a nonsmooth surface subsoiler to assess the tillage resistance and observe the adhesion phenomenon.

[figure(s) omitted; refer to PDF]

3. Results and Discussion

3.1. Results and Discussion of the Field Experiment

The laser particle size analyzer (BT-9300st) was used to analyze the content of sand, silt, and clay in each soil. The average content of each soil particle of soil is shown in Table 1.

Table 1

Mass percentage of each soil particle (%).

Soil selection	Soil particle
	Sand	Silt	Clay
Shijiazhuang city	69.82	19.41	10.77
Gongzhuling city	35.42	44.71	19.87
Siping city	31.42	41.25	27.33
Changchun city	28.12	39.82	32.06

According to Figure 9 [27–29], based on the ISSC, the soil in Shijiazhuang City, Hebei Province, is sandy loam, the soil in Gongzhuling City, Jilin Province, is clay loam, and the soil in Siping City and Changchun City, Jilin Province, is loamy clay.

[figure(s) omitted; refer to PDF]

After drying with a vacuum dryer, the dry basis soil moisture content in each test field was obtained. The soil moisture content in Shijiazhuang City, Hebei Province, was 12.3%, in Gongzhuling City, Jilin Province, was 15.6%, in Siping City, Jilin Province, was 24.3%, and in Changchun City, Jilin Province, was 19.8%.

The adhesion phenomenon of the subsoiler during subsoiling is shown in Figure 10. As illustrated, it can be seen that the sandy loam in Shijiazhuang City, Hebei Province, with a moisture content of 12.3%, and the clay loam in Gongzhuling City, Jilin Province, with a moisture content of 15.6% did not have an obvious adhesion phenomenon, and the loamy clay in Siping City, Jilin Province, with a moisture content of 24.3% and the loamy clay in Changchun City, Jilin Province, with a moisture content of 19.8% had an obvious adhesion phenomenon.

[figure(s) omitted; refer to PDF]

The objective of subsoiling desorption is to reduce drag. This study measures the degree of soil adhesion by analyzing the tillage resistance of the subsoiling shovel. When the tillage depth was 30 cm and the tillage speed was 1 m/s, tractor traction resistance during subsoiling tillage is shown in Figure 11(a). As can be seen from Figure 11, the tillage resistance of the experimental fields in Shijiazhuang City, Hebei Province, and Gongzhuling City, Jilin Province, where soil adhesion was negligible, was notably lower compared to those in Siping and Changchun, Jilin Province, where adhesion was prevalent. By selecting valid data intervals and calculating mean values, we obtained average resistances of 3.13 kN for Shijiazhuang, 3.97 kN for Gongzhuling, 6.03 kN for Siping, and 6.82 kN for Changchun. To further explore the impact of varying tillage depths, measurements were taken at 35 cm and 40 cm with a constant speed of 1 m/s. The results are shown in Figure 11(b). From the figure, it can be seen that in sandy loam and clay loam with a low moisture content, the subsoiler basically did not occur soil adhesion, and obvious adhesion occurred in loamy clay with a high moisture content. In addition, the resistance of subsoiling operation with adhesion was significantly higher than that of subsoiling tillage without adhesion.

[figure(s) omitted; refer to PDF]

Soil conditions are very complex, resulting in uncertainty in the movement state of soil particles. The force of the subsoiler blade on the soil during subsoiling can be simplified to the action of a two-sided wedge on the soil. As shown in Figure 12, the action of the two-sided wedge in the soil is divided into the following two processes: compaction and fracture, that is, the compaction and lifting of the soil particles. In an ideal state, when the wedge moves at a low speed $v_m$, the two-sided wedge moves from $A_1{B}_1\text{\hspace{0.17em}to\hspace{0.17em}}{A}_2{B}_2$, and the soil particles are lifted from $C_1\text{\hspace{0.17em}to\hspace{0.17em}}{C}_2$ by the two-sided wedge. The angle of the two-sided wedge surface of the two-sided wedge is α, the translation distance is $L_1$, the depth of action of the wedge on the soil is H, the movement distance of the soil particles is $L_2$, the included angle of φ between $C_1{C}_2$ and the normal of the wedge $A_1{B}_1$ is the external friction angle, τ is the shear stress between the soil particles, and β is the inclination angle of soil failure slip surface.

[figure(s) omitted; refer to PDF]

As shown in Figure 12, α + φ + β = 90°. Therefore, there are two ways to form soil adhesion, which are to increase the angle of wedge insertion into the soil on both sides α or increase the external friction angle φ. Both of these methods enable β to reduce to zero, causing the soil to move together with the wedge.

We simplify the motion model between the soil and the wedge, as shown in Figure 13. When the wedge angle α₁ is smaller, the cohesion between the soil particles is greater than the friction between the soil and the wedge, and the soil-sliding friction occurs at this time. When the wedge angle ${\alpha }_2$ is greater, the cohesion between the soil particles is less than the friction between the soil and the wedge; at this time, the soil undergoes internal friction. When internal friction occurs, part of the soil with the contact surface together with the movement and then in the contact surface of the adhesion phenomenon and even the formation of adherent soil nuclei. This theoretical analysis model is basically consistent with the phenomenon in Figure 10. The cohesion between the soil particles reflects the internal friction characteristics of the soil, and the sliding friction between soil particles and materials reflects the external friction characteristics of the soil.

[figure(s) omitted; refer to PDF]

3.2. Results and Discussion of the Soil Repose Angle and the External Friction Angle

Soil adhesion is determined by both internal and external friction properties of the soil. The cohesion among soil particles embodies the internal friction, whereas the sliding friction between the soil and wedges reflects the external friction characteristics [22]. The soil repose angle is the external expression of the internal friction properties of the soil and reflects the internal friction properties of the soil. Conversely, the soil external friction angle is the surface inclination of the object when the soil slides on it, which reflects the external friction characteristics of the soil. Both the soil repose angle and the soil external friction angle are affected by the type of soil particles, particle size, moisture content, and other factors.

3.2.1. Result of Soil Texture

In a similar manner, the content of soil particles with different sizes was analyzed by the laser particle size analyzer (BT-9300ST), and the mass percentages of sand, silt, and clay particles in the five textured soils are depicted in Figure 14. Based on the ISSC of Figure 9, the selected soil samples from the Heilongjiang forest humus layer soil, Henan yellow soil, Jilin black soil, Sichuan yellow soil, and Yunnan red soil were loamy sand, sandy loam, loam, silty loam, and loamy clay separately.

[figure(s) omitted; refer to PDF]

3.2.2. Results and Discussion of the Soil Repose Angle

The material accumulation test was conducted to measure the angle of repose [30]. Through the soil accumulation test, the repose angle of each soil texture when the moisture content is 0%, 10%, 20%, 30%, 40%, and 50% is shown in Figure 15. From the figure, it is evident that the repose angle of each textured soil in loamy sand, sandy loam, loam, silty loam, and loamy clay increased and then decreased with the increase of moisture content. This indicates that the strength of soil interparticle agglomeration increased and then decreased with the increasing moisture content. The peak size of the repose angle of each soil texture becomes larger with the increase of the content of clay particles in the soil. The region of peak occurrence shifted to the right with the increase of clay particles in soil texture. Consequently, an increase in the content of clay particles in the soil enhances the ability of the soil to bind water and enhances the soil particle aggregation force.

[figure(s) omitted; refer to PDF]

Assuming that the soil particles are equal-sized spheres, the model of water film formation between the soil particles is shown in Figure 16. According to Laplace’s formula, the additional pressure Δp within the water film due to surface tension is as follows:$$\begin{array}{}\text{(1)}& ∆p=\sigma \frac{1}{r_1}+\frac{1}{r_2},\end{array}$$where $r_1$ is the radius of the concave surface, taken as positive; $r_2$ is the radius of the convex surface, taken as negative; and σ is the surface tension of water. At this time, the cohesive force F between the soil particles is as follows:$$\begin{array}{}\text{(2)}& F=∆p\pi r_2^2+2\pi R\sigma \sin \alpha +\beta \sin \alpha ,\end{array}$$where R is the radius of the soil particles; α is the water film clamp angle; and β is the wetting angle of water on the surface of soil particles [22].

[figure(s) omitted; refer to PDF]

As illustrated in Figure 16, when the soil moisture content is very low, the soil particles can be in direct contact with each other; at this time, the soil particle spacing decreases to zero, that is, d = 0; with the increase of the soil moisture content, the soil particles gradually form a water film between the soil particles, d ≠ 0, the water film interface is moving upward, $r_1$ is decreasing, $r_2$ and α are both increasing, the wetting angle β is the intrinsic property of the soil water, and therefore the adhesion force F between the soil particles gradually increases with increasing the soil moisture content; when the soil water content exceeds the liquid limit, at this time, the soil particle spacing d is very large, and as the soil moisture content continues to increase, the stronger the soil mobility, cohesion gradually weakened. From (2), it is evident that as the soil particle size decreases, the cohesion between individual soil particles also decreases. However, the self-weight of soil particles also decreases, and the ratio of cohesion to self-weight increases. Consequently, the smaller the soil particles, the stronger the soil as a whole absorbs water, the stronger the soil cohesion exhibited, the larger the peak of the soil angle of repose, and the peak occurs in intervals with higher water content.

3.2.3. Results and Discussion of the Soil External Friction Angle

The soil external friction angle reflects the soil adhesion characteristics [31], and the soil external friction angle can be measured through the soil inclination experiment [23] and analyzed in comparison with the soil repose angle. The analysis results are depicted in Figure 17. The changing trend of the soil external friction angle for the five textures is similar to that of the soil repose angle. Specifically, as the moisture content increases, the soil external friction angle initially increases before decreasing. However, the peak of the external friction angle occurs in a region that lags behind the peak of the repose angle region. Specifically, when the moisture content is low, the soil repose angle exceeds the external friction angle. Conversely, when the moisture content is high, the soil rest angle is smaller than the soil external friction angle.

[figure(s) omitted; refer to PDF]

When soil adhesion occurs on the surface of the subsoiler, it can be divided into normal adhesion force and tangential adhesion force [32], among which the tangential adhesion force has a greater influence on the tillage resistance of the tillage component. When the dry soil is in contact with the surface of the subsoiler, there is no normal adhesion force, and the tangential slip friction force $F_f$ satisfies Coulomb’s friction law [22].$$\begin{array}{}\text{(3)}& F_f=\mu N,\end{array}$$where N is the normal load; μ is the coefficient of friction between the soil and the metal surface.

With increasing moisture content, the soil gradually shows obvious adhesion; at this time, the tangential friction F between the soil and the material consists of two components, which are the slip friction $F_f$ of the soil and the slip resistance $F_a$ between the soil-materials due to the adhesive force, and they are related as follows:$$\begin{array}{}\text{(4)}& F=F_f+F_a,\text{(5)}& F_a={\mu }_1{F}_b,\end{array}$$where μ is the proportional coefficient; $F_b$ is the adhesion between soil materials.

From equation (5), it is evident that soil adhesion can increase the characteristic friction coefficient of soil, resulting in an external friction angle φ increase, thereby as β as decreases, it is more likely to form soil adhesion.

The adhesion of soil to the surface of other objects is a complex interfacial reaction involving a variety of factors [33]. The classification of adsorption can be categorized into physical and chemical adsorption when the adhesion force between soil and materials occurs due to adsorption. For physical adsorption, assuming that the soil-material contact surface is circular with a water film as the medium, the adhesion force $F_b$ is known from McFarlane’s formula as follows:$$\begin{array}{}\text{(6)}& F_b=4\pi \text{RT}\cos \theta ,\end{array}$$where 4πR is the perimeter of the contact surface; T is the surface tension of the water; and θ is the wetting angle.

From (6), it is evident that the adhesion force escalates with an increase in the perimeter of the contact surface, implying a direct correlation with the area of the contact surface. As depicted in Figure 18, when the moist soil is in contact with the metal surface, the adhesion phenomenon between the soil-water film-metal surface with the water film as medium. When the soil moisture content is below the soil saturated moisture content, with increasing moisture content, the water film contact area gradually becomes larger and even forms a continuous water film, which changes from point contact to surface contact, increasing the actual contact area, and the soil adhesion force also increases, which is consistent with McFarlane’s formula. The existence of an adhesive water film is the key to the adhesion of the soil. This is consistent with McFarlane’s formula, and the existence of an adherent water film is the key to soil adhesion.

[figure(s) omitted; refer to PDF]

When the soil moisture content surpasses the saturation point, the water film also exhibits a lubrication effect. Moreover, with the increase in moisture content, the effect of lubrication of the water film becomes increasingly obvious. Thereafter, due to the weakening of soil cohesion, it is difficult for the soil to form large agglomerates, showing that soil adhesion begins to decrease with the continued increase in soil moisture content, which is consistent with the actual test results in Figure 17.

3.3. Results and Discussion of the Soil Bin Test

Using a laser particle size analyzer (BT-9300ST), the particle content of each particle size was analyzed in the soil, and the soil used for the indoor soil bin test was clay loam. When the soil moisture content was 40%, the subsoiler with a convex structure, concave structure, or smooth surface had an adhesion phenomenon, but the adhered soil did not agglomerate into a large number. When the soil moisture content is 30%, the national standard subsoiler with a smooth surface and the bionics subsoiler with concave structure also have adhesion phenomenon, and a large amount of soil particles agglomerated in a whole adhering to the subsoiler; for the subsoiler with convex structure, only a litter of soil agglomerates adhering. When the soil moisture content is 20% or 10%, the three types of subsoilers do not have an obvious adhesion phenomenon. The adhesion phenomenon of soil with different moisture content is consistent with Figure 17(d).

When the tillage depth was 40 cm and the speed was 1 m/s, the tillage resistance of three types of subsoilers at 30% soil moisture content is shown in Figure 19(a). By changing different tillage depths, the tillage resistance at tillage depths of 30 cm and 35 cm and speed of 1 m/s was measured, and the average value of the stable interval was taken. The results are shown in Figure 19(b). The results indicate that the tillage resistance of the smooth surface subsoiler and the concave structure subsoiler are similar and both larger than the convex structure subsoiler obviously. The average value of the smooth subsoiler at the tillage depth of 30 cm, 35 cm, and 40 cm are 3.22 kN, 5.02 kN, and 6.36 kN, respectively. The average value of the convex subsoiler at the tillage depth of 30 cm, 35 cm, and 40 cm are 2.67 kN, 4.12 kN, and 5.70 kN, respectively. Compared to the smooth subsoiler, the tillage resistance of the convex subsoiler is reduced by 17.08%, 17.93%, and 10.38%, respectively. The average drag reduction is 15.13%.

[figure(s) omitted; refer to PDF]

When the moisture content of the clay loam soil reaches 30%, a serious adhesion phenomenon occurs in the subsoiler. At this time, the apparent external friction F between the soil material can be specifically divided into two parts, one is the slip friction $F_f$ of the soil and the other is the slip resistance $F_a$ resulting from the adhesion between the soil material. They are the same as equation (4), which has the following relationship:$$\begin{array}{}\text{(7)}& F=F_f+F_a={\mu }_{1}N+p·S=\mu N,\end{array}$$where ${\mu }_1$ is the proportion coefficient, N is the normal load, p is the adhesion force, and S is the adhesion area of the actual contact.

The normal load N is mainly composed of two parts as follows:$$\begin{array}{}\text{(8)}& N=N_1+N_2,\end{array}$$where $N_1$ is the positive pressure generated by the self-weight of the soil and mutual extrusion between soil and material and $N_2$ is the pressure difference between the upper and lower surfaces of the soil due to atmospheric pressure, which can be expressed by the vacuum degree of the soil-material contact surface, which affects the size of the soil adhesion.

The interaction model between the convex structure of the subsoiler surface and the soil is illustrated in Figure 20. The convex structure compresses with the soil in the process of advancing, forming a large stress difference with the nonextrusion area, and the large aggregates are broken due to cracks caused by stress concentration [34]. At this time, the soil gas quickly skims the subsoiling shovel contact surface from the crack, which greatly reduces the vacuum between the soil material contact surfaces. Consequently, this reduces soil adhesion, demonstrating the antiadhesion effect.

[figure(s) omitted; refer to PDF]

4. Conclusions

Based on the internal and external friction characteristics of the soil, this study measured and analyzed the relationship between the soil repose angle and the external friction angle. In addition, the study derived the conditions under which soil adhesion occurs by considering the soil adhesion mechanism. The effectiveness of the convex structure subsoiler to antiadhesion was verified. The specific findings are as follows:

(1) When investigating soil adhesion, compared to the traditional angle of internal friction of soil, measuring the soil repose angle is simple and efficient and can fully reflect the internal friction characteristics of the soil. However, when the soil moisture content reaches 50% or above the liquid limit, the soil repose angle measurement is inaccurate due to serious soil collapse.

(2) The texture of the soil and the moisture content greatly influence the angle of repose and external friction. When the soil repose angle is greater than the external friction angle, the soil exhibits greater cohesion than the external friction, making it less prone to adhere; when the soil repose angle is smaller than the external friction angle, the soil shows less cohesion than the adhesion force, making it more susceptible to adhere.

(3) Sandy soil, which has a low clay particle content, exhibits weak binding force between soil particles and water, resulting in minimal cohesion and almost no adhesion characteristics. Therefore, adhesion does not easily occur when the sandy soil comes into contact with the surface of the subsoiler. In clay soil with a moisture content of less than 20%, due to the adhesion of water film formation difficulties, the soil and subsoiler surface is difficult to form a continuous contact surface, showing weak adhesion characteristics. However, when the moisture content exceeds 20% but remains below the liquid limit, a water film forms among the soil, water, and metal surfaces, leading to strong adhesion characteristics. The adhesion of the soil significantly affects the tangential resistance of the soil-engaging components.

(4) Compared to a smooth surface subsoiler, the convex surface structure subsoiler reduces tillage resistance by 12.61% in clay loam with a moisture content of 30%. When the nonsmooth surface structure of the subsoiler moves relative to the soil, the convex structure hinders the movement of the local soil, which makes the whole piece of soil unevenly stressed, forming a stress concentration area and then breaking the soil block. After the soil block breaks, the soil gas can quickly enter the soil contact surface, greatly reducing the vacuum between the soil-material contact surface and then reducing the soil adhesion force, showing a good effect of antiadhesion.

(5) From an economic standpoint, when tilling dry or sandy soil, the subsoiler surface is not prone to adhere, and the surface structure is subject to significant wear. Therefore, a smooth surface subsoiler can be used to reduce processing costs. However, when tilling in clay soil with high moisture content, adhesion is likely to occur. In this case, the convex structure on the nonsmooth surface of the subsoiler effectively reduces drag and energy consumption. Hence, a subsoiler with a convex structure is recommended in clay soil with a high moisture content to minimize energy consumption [35].

Authors’ Contributions

J.L. conceptualized, validated, and visualized the study, curated the data, wrote the original draft, and contributed to software; J.L. and Y.M. developed the methodology; J.L. and H.Q. contributed to formal analysis and reviewed and edited the study; J.L. and B.W. investigated the study; P.G. and Y.M. contributed to resources; Y.M. supervised the study, administered the project, and contributed to funding acquisition. All the authors have read and agreed to the published version of the manuscript.

Acknowledgments

This research was funded by the National Natural Science Foundation of China, grant number 52275288; the Jilin Province Science and Technology Development Plan Item, grant number 20210202021NC; and the Changchun Science and Technology Development Plan Item, grant number 21ZGN15.

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