# Research Progress and Engineering Applications of Viscous Fluid Mechanics
## 2 Advances in the Study of the State of Motion of Viscous Fluids
### Experimentation and Research
#### 2.1.2 Experiments and Studies on Droplet Impacts in Viscous Fluids
[50], based on high-speed micro-digital camera technology and the establishment of the finite element numerical model of droplet impact on a superhydrophobic wall, carried out experiments to study the dynamic behavior of Newtonian fluid droplets with different viscosities (0.9-27.7 m Pa\(\cdot\)s) impacting on a superhydrophobic wall (the static contact angle is 158\({}^{\circ}\)). Figure 5 shows the experimental process of the impact of Newtonian fluid droplets with different viscosity on the superhydrophobic wall captured by the high-speed camera when the We number is 28.2. It can be seen that the performance of the high-speed camera can observe a very clear droplet collision process. On the issue that the impact velocity, angle, and impact energy of droplets have important effects on the experimental results, [PERSON] et al. [51] investigated the impact kinetic behaviors of water, anhydrous ethanol, and low-surface-energy droplets of different viscosities on hydrophobic surfaces through experimental visualization and explored and analyzed the effects of the three influencing factors, namely, the impact velocity, the surface wettability, and the droplet viscosity, on the impact kinetics of viscous droplets. The impact dynamics of viscous droplets were explored and analyzed. [PERSON] et al. [52] studied the impact process of viscous liquid and dense particle jets against a wall with a background of gasifier gasification technology and concluded that when the particle size is small, the dense particle impact flow shows a particle film morphology similar to the liquid impact; the higher the solid content rate, the lower the particle rebound probability and the more homogeneous the velocity and energy of the particles, which makes it easier to present the particle film morphology and other conclusions. In order to analyze the process of collision aggregation of unequally large droplets on walls with different wettability, [PERSON] et al. [53] prepared unequally large droplets with a volume ratio of 1:2 and carried out collision experiments in which small droplets on substrates with different wettability hit large droplets from different contact angles using high-speed camera technology to analyze the kinetic behaviors of droplet collision aggregation. The experimental device is shown in Figure 6, which provides a new idea of device construction for droplet collision experiments. Figure 5: Snapshots of Newtonian fluid droplets with different viscosities impacting on the superhydrophobic surface at _We_ = 28.2 (all of the scale bars represent 1 mm): (**a1–a4**) show the process of a 50 wt.% glycerol droplet impacting the superhydrophobic wall, and (**b1–b4**) show the process image of the initial state, 5.0 ms, 6.5 ms, and 7.0 ms, respectively, after the process of a 60 wt.% glycerol droplet impacting the superhydrophobic wall [50]. There exists the problem that the droplets cannot be completely wetted when contacting the solid surface, or there exists a large contact angle, which affects the deformation and rebound behavior of the droplets, making it difficult for the experimental results to accurately reflect the theoretical model. [PERSON] et al. [54] used a high-speed shadow imaging technique to investigate the evolution of viscous droplets (glycerol aqueous solution) impacting the drying and pre-wetting mesh surface to form liquid fingers and fragmentation and concluded that an increase in droplet viscosity and a decrease in mesh width would inhibit the complete penetration of droplets into the drying mesh surface and so on. [PERSON] et al. [55] used a high-speed camera to set up an experimental device to study the different changes in droplets impacting plates with different angles and humidity at different We numbers, and their experimental results provide some reference bases for complex fluid motion to a certain extent.