Anisotropic wetting characteristics of droplet on micro-grooved surface

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 (xi = L-perpendicular to/ L-parallel to) was linearly correlated with the droplet wetting shape parameters (NRWR, NGWG, sin theta(perpendicular to) and sin theta(parallel to)) 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 perpendicular to L parallel to/R-2) was corre-lated with the droplet wetting shape parameters (N-G/N-R, sin theta(perpendicular to) and sin theta(parallel to)) in power function. The relative de-viations between the experimental results and the calculated results were smaller than 20%. The normalized free energy (E * = E/V-2/3/gamma(lg)) 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, W-G/W-R) 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 xi, A * and E * were obtained, which are conducive to further under-standing the anisotropic wetting characteristics of droplet on grooved surface.

Automated summary

The anisotropic wetting characteristics of droplets on micro-grooved surfaces were studied through experiments and simulations. The results showed that gravity played an important role in affecting droplet wetting behavior without inducing collapse of the Cassie-Baxter state. The spreading stage of the deposited droplets was dominantly influenced by capillary forces, and anisotropic wetting characteristics were evident with differences in droplet base length and contact angle in different directions.