## 2 Materials and Methods
### Experimental Setup  
The structure of the disc is shown in Figure 1a. All structures of the disc (diameter = 130 mm) were designed using CAD software UG 8.5, including the inlet and outlet of the chip, spiral microchannels, and oil storage chamber. The specific manufacturing process of the chip is described as follows. Firstly, the microchannels and chamber were machined on the PMMA substrate with a thickness of 3 mm using a precision CNC milling machine. Then, the two processed PMMA plates were placed in the Ultrasonic Cleaning Machine (JT-1027 HT; Jietuo, Shenzhen, China) and Plasma Cleaner (PDC-002; Harrick Plasma, Ithaca, NY, USA) for surface treatment, lasting for 10 min. Finally, the disposed PMMA plates were placed in the thermocompression border (WH-2000C; Wenhao, Suzhou, China) with the following working conditions. The pressure is set as 0.25 MPa, temperature is set as 100 \({}^{\circ}\)C, and the running time is 1 h. The microchannels of the chip were hydrophobically treated with CLEMENS (the surface interaction in the case of a hydrophobic PMMA surface with deionized water and mineral oil are shown in Appendix A). The simple centrifugal microfluidic chip with spiral microchannel was fabricated by the above method. And the parameters of the spiral microchannel are listed as follows. The spiral radius ranges from 35 mm to 55 mm. The change rate of the spiral radius is 0.637, 0.796 and 1.061 mm/rad, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Density/g: mL** & **Viscosity/mPa-s** & \multicolumn{1}{c}{**Surface**} \\  & & & **Tension/mN:m\({}^{-1}\)** \\ \hline DI water & 1 & 1 & 72.0 \\
3\# Oil & 0.827 & 12.60 & 25.9 \\
5\# Oil & 0.820 & 18.11 & 30.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical properties of materials. respectively. The cross-section is a rectangle with an aspect ratio of 1:1 and hydraulic diameter of 0.5 mm. The whole centrifugal microfluidic system is shown in Figure 1b. Its running process is depicted as follows. The centrifugal microfluidic chip and a marking disk were connected to the motor coupling, which was driven by a brushless DC motor (80 BL89S50-445 TK0; Shidaichaoqun, Beijing, China). At the same time, the motor speed was controlled by servo driver (ZM-6615; Shidaichaoqun, Beijing, China) and STM32 MCU. The high-speed camera (FASTCAM Mini UX; Photron, Japan) and other imaging modules capture the flow state of the droplet in the rotary disc. The specific experimental operation is described as follows. Prior to centrifugation, the mineral oil was filled into the oil storage chamber (about 0.5 mL) and spiral microchannel (about 0.21-0.35 mL) by a syringe. Then, the droplet with diameter of 200-300 \(\upmu\)m selected by a metallographic microscope (15 JF-V; CSOIF, Shanghai, China) was injected into the chip, and adjusted to mark 1 with a syringe. Finally, the motor could be started after the chip and the marking disc were fixed with the motor coupling. At the same time, a high-speed camera was used to record the flowing process of the droplet in the microchannel, and the camera parameters were set as: shooting speed 4000 fps, shutter speed 1/50,000 s. The images collected by the camera were analyzed by Photron-FASTCAM software to obtain the physical quantity of droplet flow. The obtained flow state of the droplet is shown in Figure 2. In the pictures, the time of flash photo group is the frame time corresponding to each photo at the shooting time, i.e., the time difference of droplet movement. Based on the marks on the pictures, the flow positions of the droplet in the microchannels at different times can be obtained, and the flow distance of the droplet can be calculated. We record the flowing distance of the droplet in 1 s and obtain the average velocity, which can be approximated as equal to the instantaneous velocity at the midpoint of the segment due to the short time and slow change of velocity.