On the collective impact of defects on the shape of advancing three and four-phase contact lines.*

This study considers the impact of a series of macroscopic defects on a solid surface on an advancing contact line (CL) to understand collective effects on interface shape, depinning force (F_d) and energy (E_d).
Macroscopic defects were produced on stainless steel (s/s) in the form of regular holes and cocoa butter (cb) patches and by depositing cb patches on glass slides. Samples were submerged into DI water and EtOH aqueous solutions. The defects varied in radii (r) (0.27-1.44mm) as well as number. The distance between defects (d) varied in the range 0.9-2.7mm The contact angles of the liquids on s/s ranged from 50-95°, 10-30° on glass and 70-110° on cb.
The depinning depth (h_d) and the position of the CL (h_m) depend on d. Cumulative effects of the holes lead to greater h_d and h_m. Cb defects showed lower h_d and h_m, which can be explained by the ability of the four-phase CL to move on the surface of the defects, as opposed to the complete pinning observed at the base of the holes. This justifies also the lower F_d and E_d of cb defects compared to holes of same size. A linear dependence of the F_d was observed with n r (n is the number of defects), while E_d was proportional to n r^2, but lowering γ decreased h_d , F_d and E_d in a non-linear way.