## 1 Introduction  
Micro-droplet is a type of fluid movement based on microfluidics, which usually exists in the discontinuous distribution of one liquid phase in another insoluble fluid phase [1]. In the droplet-based microfluidic field, its common forms include: water-in-oil (W/O), oil-in-water-in-oil (O/W/O), water-in-oil-in-water (W/O/W) and so on [2; 3; 4]. In practice, each discrete droplet could act as an individual reaction, which can not only reduce sample consumption and avoid cross-contamination, but also realize high-throughput parallel reaction. Therefore, all the prominent features brought about by droplet-based microfluidic techniques are contributing to this hot research topic and increased applications in many fields, such as drug delivery, cell research, chemical analysis disease detection, etc. [5; 6; 7; 8; 9; 10]. However, the connection and realization of different functions in the droplet-based microfluidic chip depend on the droplets' driving method and flow form. Therefore, droplet manipulation is a critical technical problem in realizing these application fields. Studying droplet driving techniques and movement under driving is of great significance.  
Centrifugal drive [11] is one of many droplet driving methods. Compared with other driving methods, it does not need to rely on external high-precision equipment like surface tension drive [12; 13; 14]; only a simple motor can drive it. It is also not as sensitive to the physical/chemical properties of liquids as magnetic drive [15], which can be used to drive biological fluids such as blood, urine, and organic solvents. More recently, research on droplet centrifugal drive has mainly focused on the combination of centrifugal force field and special structure: [PERSON] et al. [16] designed a circular chip with multiple radial channels. The sodium alginate solutions in radial channels were flung into CaCl\({}_{2}\) solutions in the form of droplets under centrifugal force, resulting in fabrication of oblate spheroidal calcium alginate particles. [PERSON] et al. [17] reported a microfluidic pressure regulator scheme, along with the use of microcapillaries to periodically generate droplets, and also demonstrated droplet-based cell transfection and perovskite synthesis in the centrifugal platform. However, most of these studies focused on the combination of droplet-based microfluidics and centrifugal drive involve the droplet generation process; few studies have been carried out on droplet manipulation, namely the flow of the droplet driving process.  
To make up for this deficiency, we proposed a centrifugal driving method with a spiral microchannel in this study. This driving mode is simple in structure, convenient in preparation, and less sensitive to the physical/chemical properties of the drive fluid. It can also be combined with various injection methods, such as the pre-storage in oil reservoir, the non-contact injection method proposed by [PERSON] et al. [18]. We prepared a simple centrifugal microfluidic chip with a spiral microchannel and made the chip rotate under the centrifugal force. The kinematic analysis of the movement of the W/O droplet was carried out under this driving mode and chip structure. The velocity formula was obtained and compared with the experimental results. The aim is to understand how to control droplet velocity and eventually apply this knowledge to different applications in the future (flow droplet PCR, particle sorting, etc.).