The dependence of microdroplet size on the parameters governing the dewetting process on circular micropillar arrays

We theoretically investigated the correlation among the size of microdroplet, the geometrical parameters of circular micropillars, and the parameters governing the dewetting process. Dimensional analysis is used to determine independent dimensionless groups to characterize the diameter of microdroplet, i.e., the Ohnesorge number (Oh), the capillary number (Ca), the dimensionless liquid thickness (H), and the contact angle (θ). The simulation results show that the size of microdroplet on the top surface of micropillar depends on the parameters in the dewetting process. The dimensionless droplet diameter (d) grows with the increase of H, where d becomes saturated provided H reaches a critical value of 2.5. The decrease of Ohnesorge number results in microdroplets with large diameter. For small contact angle, the top surface of micropillar is mostly wetted, which leads to large microdroplets. When the value of the capillary number increases, the viscous force on the top surface of micropillar magnifies, and leads to the growth of droplet diameter until the value of Ca reaches a critical value. For H < 2.5, the magnitude of the critical capillary number (Ca^c) depends on Oh, θ, and H*, while Ca^c is a constant for H ≥ 2.5. The dimensionless droplet diameter (d) can be determined by the product of functions of Oh, θ, H*, and Ca*.