Many-body dissipative particle dynamics simulation of the anisotropic effect of droplet wetting on stripe-patterned heterogeneous surfaces.

The droplet anisotropic wetting on chemically heterogeneous stripe-patterned substrates after impact is studied by using many-body dissipative particle dynamics in this work. After a low-velocity impact on substrates with different length ratio (ratio of stripe width to the initial droplet size) and Cassie area fraction, the droplet can show various shapes and contact angles in parallel and orthogonal directions to the stripes. The elongation of the droplet can be increasingly evident when increasing the length ratio. Also, the contact angles at both directions follow the Cassie-Baxter predicted values well at low length ratio while deviate from them at high length ratio. Both the capillary and kinetic effects have a significant influence on the anisotropic wetting. When impact with higher velocity, the droplet leaves some residues on the hydrophilic stripes of the substrates. Surprisingly, the residues for substrates with certain length ratio (≤0.316) are distributed in circular regions with almost identical radii, indicating the spreading stages of them are independent on the surface properties and dominated by the kinetic effect. When the droplet reaches a stable state, the main body shows anisotropic wetting behavior, indicating the capillary effect dominates the wetting instead of the kinetic effect at the retraction stage. Unlabelled Image • Combined effect of length ratio, Cassie area fraction and initial kinetic energy is considered in spreading on substrates. • Critical length ratio is identified to separate the anisotropy and isotropy of wetting. • The droplet shows no anisotropic effect in spreading stage when given a high initial impact velocity. • A simple algorithm is proposed to calculate the contact angle of the droplet with distorted contact line. [ABSTRACT FROM AUTHOR]