2.5.1. Restitution Coefficient between Soybean Seed Particles and Boundary

A drop test [24,25] was used to measure the restitution coefficient between the soybean seed particle and boundary. Using SN42 as an example, the soybean seed particle is placed vertically at the vacuum nozzle along its length and marked directly in front of the seed particle, as shown in Figure 4a–c. A high speed camera is used to record the above test procedure, while ensuring that the soybean seed particle is moving in a vertical direction. The software accompanying the high-speed camera was used to analyse the experimental results. Neglecting air resistance, the restitution coefficient was calculated as follows:

$$
\epsilon = \sqrt{\frac{h\_1}{h\_0}}\tag{4}
$$

where *h*<sup>0</sup> is the distance between the initial position and coordinate origin of soybean seed particles, as shown in Figure 4b, and *h*<sup>1</sup> is the distance between the highest point and coordinate origin of the first vertical rebound of soybean seed particles, as shown in Figure 4c.

**Figure 4.** PCC image analysis process: (**a**) coordinate origin selection and distance calibration, (**b**) the distance between the initial position and coordinate origin of soybean seed particles, and (**c**) the distance between the highest point and coordinate origin of the first vertical rebound of soybean seed particles.

#### 2.5.2. Restitution Coefficient between Soybean Seed Particles

A single pendulum collision test [26,27] was used to measure the restitution coefficient between soybean seed particles. A soybean was lifted with a straightedge to the position shown in Figure 5a–c. The software that accompanied the high-speed camera was used to analyse the experimental results. The equation for the restitution coefficient, ignoring air resistance, was as follows:

$$e = (\sqrt{h\_1} - \sqrt{h\_2}) / \sqrt{h\_0} \tag{5}$$

where *h*<sup>0</sup> is the vertical distance between the initial position of soybean seed particles and coordinate origin, as shown in Figure 5b, and *h*<sup>1</sup> and *h*<sup>2</sup> are the distances between the highest point of the vertical rebound of the two soya beans and the origin of the coordinate after the first collision, respectively, as shown in Figure 5c.

**Figure 5.** PCC image analysis process: (**a**) coordinate origin selection and distance calibration, (**b**) the vertical distance between the initial position of soybean seed particles and coordinate origin, (**c**) the distances between the highest point of the vertical rebound of the two soya beans and the origin of the coordinate after the first collision.

### *2.6. Static Friction Coefficient of Soybean Seed Particles*

#### 2.6.1. Static Friction Coefficient between Soybean Seed Particles and the Boundary

The method for measuring the static friction coefficient between the particles and boundary was the slope method [28,29]. Three soybean seed particles with an intact appearance were fixed on a small square glass plate with glue. The boundary material and inclinometer were fixed to the slope meter and the test material was placed on the boundary material, as shown in Figure 6. For the trials, three test specimens of each variety were used to produce three replicate experiments for each specimen. The specimens were made with the soybean in a random orientation with no fixed direction. The formula for the coefficient of static friction between the soya seed pellet and the boundary is shown below:

$$
\mu\_{SP-B} = \texttt{tarn} \tag{6}
$$

where *μSP*−*<sup>B</sup>* is the static friction coefficient between the soybean seed particle and boundary and *α* is the indication of the inclinometer when the test specimen is just sliding, rad.

**Figure 6.** Static friction coefficient between soybean seed particle and boundary measured by the slope method.

2.6.2. Static Friction Coefficient between Soybean Seed Particles

The slope method was used to measure the static friction coefficient between soybean seed particles. The test specimens were made by fixing three intact appearing soybean seed particles to a small square glass piece with glue. We fixed one specimen on the inclinometer and placed the other on top, to ensure that the two test soybean particles were in exact vertex contact, as shown in Figure 7. The equation for calculating the static friction coefficient between soybean seed particles is as follows:

$$
\mu\_{SP-P} = \tan \beta \tag{7}
$$

where *μSP*−*<sup>P</sup>* is the static friction coefficient between soybean seed particles and *β* is the indication of the inclinometer when the test specimen is just sliding, rad.
