Abstract
Maneuvering the droplet spreading by changing the wettability of solid surface with surface treatment technology has attracted much attention. Smoothed Particle Hydrodynamics (SPH) is a meshfree Lagrangian particle method and has special advantages in coping with large deformations and interfaces problems. In this paper, a multiphase SPH model was proposed which is associated with number density to approximate density, color function to calculate stress at the interface between different phases, and particle shifting technique to regularize particle distribution. The multiphase SPH model was first validated and then applied to numerically study droplet spreading on surfaces with asymmetrical wettability (different surface tension coefficients on two sides). The simulation results demonstrated that the spreading characteristic is closely dependent on the ratio of the surface tension on the two sides of the asymmetric surface. According to different spreading forms, the Contact angle maps were obtained which clearly described the relationship of the surface tension ratio, the contact angles on two sides and the spreading modes. Furthermore, through analyzing the evolution of velocity at four modes (Bi-directional spreading (Left deviation), Bi-directional spreading (Right deviation), Uni-directional spreading (Left) and Uni-directional spreading (Right)), a general law of the change and the distribution of velocity is formulated. Finally, the mode of uni-directional spreading is theoretically studied and theoretical solutions which can quantitatively describe the evolution of the droplet on the substrate, while the theoretical solution is associated with two dimensionless numbers, which respectively determine the displacement and speed of the droplet, are proposed. All these results provide solid theoretical basis and profound insights for the spreading of droplets on surfaces with asymmetrical wettability.
