Anisotropic wetting characteristics of droplet on micro-grooved surface

Abstract

Anisotropic wetting characteristics of droplet play an important role in a wide applications including directional droplet transportation and microfluidics in lab-on-a-chip. The anisotropic wetting shape of droplet on micro-grooved surface in Cassie-Baxter state was studied in experiment and simulation, and the corresponding droplet free energy was calculated in simulation. The stability of Cassie-Baxter state of droplet was investigated, and the dimensionless Bond, capillary and Reynolds numbers were estimated. Results shown that the gravity played an important role in affecting the droplet wetting behavior, but would not induce the spontaneous collapse of Cassie-Baxter state. The spreading stage of the deposited droplet was dominated by capillary force. The anisotropic wetting characteristics of droplet were evidently shown as the droplet base length larger than the base width, and the droplet contact angle in the direction perpendicular to groove larger than that in the parallel direction. The droplet base aspect ratio (ξ = L⊥/ L∥) was linearly correlated with the droplet wetting shape parameters (NRWR, NGWG, sinθ⊥ and sinθ∥) and the spherical droplet radius with the same volume (R). The relative deviations between the experimental results and the calculated results by the fitting linear correlation expression were smaller than 10%. The normalized approximate droplet base area (A * = L⊥L∥/R2) was correlated with the droplet wetting shape parameters (NG/NR, sinθ⊥ and sinθ∥) in power function. The relative deviations between the experimental results and the calculated results were smaller than 20%. The normalized free energy (E * = E/V2/3/γlg) of a droplet on micro-grooved surfaces was not constant, which was decreased with the increase of ridge width. The normalized free energy was correlated with the droplet wetting shape parameters (NRWR, NGWG, WG/WR) and the parameter R in power function. The relative deviations between the actual and the fitting values of the E * were smaller than 1.5%. Furthermore, the effect way and effect degree of these droplet wetting shape parameters on the ξ, A * and E * were obtained, which are conducive to further understanding the anisotropic wetting characteristics of droplet on grooved surface.