Study on the Multifactor Coupling Effect of X80 Steel Inclined Plate Wetting Gradient Surface Promoting the Self-Movement of Underwater–Oil Droplets

To improve the oil-water separation efficiency of inclined plates in oil removal tanks, a wetting gradient was constructed on the surface of X80 steel substrates via chemical modification combined with mechanical polishing. Subsequently, three factors governing the underwater self-propulsion behavior of oil droplets on the wetting gradient surfaces were systematically investigated through single-factor experiments and response surface methodology (RSM). (1) Feasibility experiments demonstrate that underwater self-propulsion of oil droplets can be achieved both when the oleophilic surface faces upward with the oleophobic surface downward and when the oleophobic surface faces upward with the oleophilic surface downward. (2) Single-factor experiments reveal that the distance and velocity of oil droplet self-propulsion increase monotonically with increasing inclination angle while exhibiting a "first increase and then decrease" trend with the elevation of droplet volume and density. (3) Coupling effect analysis indicates that the order of significance of the three factors affecting the self-propulsion distance of oil droplets is inclination angle > droplet density > droplet volume. Additionally, the optimal parameter combination predicted by the model under the experimental conditions is an inclination angle of 58.77°, a droplet volume of 5.16 μL, and a droplet density of 0.73 g/mL.