## 3 Results and Discussion  
It can be seen from Equation (12) that the movement of W/O droplet in the spiral microchannel driven by centrifugal field is related to the physical properties of materials, the rotation speed of the motor, and the change rate of the spiral radius. In the above model, we assumed that the droplet was a rigid body. In practice, the droplet will change such as stretching/shrinking with the action of various forces. So, the deformation of the droplet needs to be considered. In this paper, the aspect ratio is used to determine the degree of deformation of the droplet, which is defined as the ratio between the length in the direction of velocity and the width in the radial direction. The change in aspect ratio of the droplet under different conditions is shown in Figure 4. It can be seen that the change of the aspect ratio of the droplet mainly occurs in the acceleration stage of the motor. More than half of the increase occurs in the first 10 s after the motor starts to rotate. A relatively obvious jumping change occurs when the droplet is close to the wall, as a result of the radial deformation of the droplet being hindered by the wall. As the droplet moves outward along the spiral microchannel with a stable rotation speed, the aspect ratio increases steadily and slowly. At a large diameter of 0.3 mm, a high-speed of 1000 rpm and a low viscosity of 12.60 mPa-s, the deformation of droplet caused by the centrifugal force, viscous force, and interfacial stress is larger. The relationship of the three forces can be expressed with the Bond number (\(Bo=\rho\omega^{2}aD^{2}/\gamma\)) and the Capillary number (\(Ca=\mu u/\gamma\)). The interfacial tension of water-PMMA is 32 mN\(\cdot\)m\({}^{-1}\), the interfacial tension of water-oil is 41 mN\(\cdot\)m\({}^{-1}\). In the current case, the Bond number ranges from 6.46 \(\times\) 10\({}^{-3}\) to 2.51 \(\times\) 10\({}^{-2}\) and the Capillary number ranges from 5.86 \(\times\) 10\({}^{-3}\) to 8.18 \(\times\) 10\({}^{-3}\). This shows that the surface tension force is dominant at the microscale. A smaller scale of the droplet has a stronger tension effect, and a larger viscosity has a stronger viscous stress, which can reduce the deformation of the droplet. When the motor stops gradually, the aspect ratio of the droplet decreases gradually and finally becomes smaller than the initial value, which indicates that the droplet is compressed. The slight compression of the droplet can be explained as an inertia effect. From the experimental results, the deformation amount of the droplet is far less than the initial size, and the minimum deformation of the droplet satisfies the rigid body assumption. The flow state of the droplet at a steady rotation speed is essential. Thus, we intercept the flow state of the droplet within 10 to 90 s after the motor starts to rotate (Figure 4). In the condition of \(a=1.061\) mm/rad, \(\omega=1000\) rpm, \(D=0.3\) mm, and \(\mu_{o}=12.60\) mPa-s, the variation of the transient contact angles and the length ratio of the contact line with time at different measurement positions is shown in Figure 5. The contact angle (\(\alpha_{1}\)) of the leading-edge interface of the droplet gradually decreased with time, while the contact angle (\(\alpha_{2}\)) of the trailing-edge interface remained relatively constant (\(\pm 1^{\circ}\)). The initial length of the contact line is 51 \(\upmu\)m, which shows a relatively uniform increase with time. These values can then be used for kinematic analysis of the model. The observed velocity of the droplet in the microchannel is compared with the theoretical velocity, as shown in Figure 6. The instantaneous velocity here is defined as the ratio between the experimentally observed position and time.