Critical sliding angle of water droplet on parallel hydrophobic grooved surface

The critical sliding angles of water droplets on grooved surfaces in parallel direction are investigated in this study. Considered the length and continuity of threephase contact line, a method to calculate the critical sliding angle of droplet in Cassie state on grooved surfaces is proposed. A definition of effective length, Lcl, equaled to the length component of three-phase contact line in the perpendicular direction to groove, is proposed to explain and compare the difference of critical sliding angles of the same volume droplets on different surfaces. The proposed method can accurately calculate the critical sliding angle, and the maximum error between calculated results and experimental results is smaller than 2.5° in the test samples. The effective length of droplet on the smooth surface is defined as the reference effective length, L0. The dimensionless effective length, Lcl / L0, is an important intrinsic factor to determine the droplet critical sliding angle, and a good positive linear correlation between the critical sliding angle and dimensionless effective length is found. Similarly, in general, the ratio of rib width to groove width, ω, also exhibits a positive correlation with the critical sliding angle. This result may be useful in a guideline to design the groove structure with a low ratio ω for enhanced water drainage.