## 2 Materials and Methods
### Physical Parameters
#### 2.2.3 Coefficient of Restitution  
The coefficient of restitution is a physical parameter that quantifies the ratio of the relative velocities of two objects after a collision relative to that same ratio before the collision. In agricultural applications, the particle coefficient of restitution is typically defined as the ratio of a particle's velocities before and after impact. This parameter is essential for understanding the motion-related behavior of particles following a collision and is primarily used for calibrating simulation parameters to model particle dynamics accurately [17]. The coefficient of restitution was determined by testing the seeds' free fall and rebound. The experimental setup consisted of material plates for testing (PMMA, SAE 304 stainless steel, PLA, and 6061 aluminum alloy), grid coordinate paper, digital camera (model: Canon EOS 200 DII), tweezers, and LED lighting. The coordinate paper was affixed to the wall perpendicular to the tested material plate, with the camera lens aligned directly toward it. The LED lighting was adjusted to ensure that the camera could capture the trajectory of  
Figure 2: Compression experiment used to test the Sesbania seeds. (1) INSTRON 6800; (2) Sesbania seed. the seed's fall, as illustrated in Figure 3. Given the small size of Sesbania seeds, the initial height was set to 100 mm to record the seed's rebound process fully. During the experiment, a single Sesbania seed was carefully positioned 100 mm above the material's surface in order to be tested using forceps, with one side of the forceps aligned closely with the coordinate paper. The seed was released to allow it to fall freely under gravity. Simultaneously, the digital camera recorded the entire process in full HD at 60 frames per second to capture the trajectory of the seed's motion in order to measure its rebound height after its collision with the material. Subsequently, the video file was imported into the video processing software and played in slow motion to analyze the collision and rebound trajectory. The coordinates of the seed at its highest rebound point were recorded to determine the maximum rebound height. Ten groups of valid rebound height data were collected for each of the four materials tested (PMMA, SAE 304 stainless steel, PLA, and 6061 aluminum alloy). Neglecting the velocity change of the test material plate after the collision [18], the coefficient of restitution was determined by applying Formula (4). \[e=\frac{\left|v_{1}^{\prime}-v_{2}^{\prime}\right|}{\left|v_{1}-v_{2}\right|}= \frac{\left|v_{1}^{\prime}\right|}{\left|v_{1}\right|}=\frac{\sqrt{2 gh}}{ \sqrt{2 gH}}=\sqrt{\frac{h}{H}} \tag{4}\]  
where \(e\) is the coefficient of restitution; \(v_{1}\) is the velocity of the seed before collision (in m/s); \(v_{1}^{\prime}\) is the velocity of the seed after collision (in m/s); \(v_{2}^{\prime}\) is the velocity of the material plate before collision (in m/s); \(v_{2}^{\prime}\) is the velocity of the material plate after collision (in m/s); \(H\) is the initial falling height of the seed (in mm); and \(h\) is the maximum rebound height of the seed (in mm). The coefficient of restitution between Sesbania seeds was also determined using free fall and collision rebound tests. Before the measurement, Sesbania seeds with similar diameters were carefully arranged in close contact, and attached to a paper surface using tweezers, forming a seed material plate (Figure 4). To enhance the visibility of the rebound height after collision, during the experiment, the Sesbania seeds were positioned at a height of 250 mm, directly above the seed material plate, and then allowed to fall freely under gravity. Ten groups of valid rebound height data were collected, and their average value was calculated. Figure 3: Experiment measuring the collision recovery coefficient: (1) tweezer; (2) Sesbania seed; (3) grid coordinate paper; (4) material plate; and (5) Canon EOS 200D II.