Advancing and Receding Motion of Droplets on Ultrahydrophobic Post Surfaces

Abstract

We have fabricated a range of silicon post surfaces where post width and spacing have been systematically varied. As one subset, we have generated surfaces where the post spacings in x and y assume different values. On these surfaces, the dynamic contact angles become anisotropic. A fluoropolymer monolayer is photochemically attached to the microstructured silicon, leading to the appearance of ultrahydrophobic properties. On one side, the advancing contact angles on these surfaces are not affected by variations in the geometric parameters. This furthers the conclusion that, during the advancing motion, a true contact angle of 180 degrees is reached. On the other side, the receding angles are strongly influenced by the post size and spacing. We quantitatively analyze this dependence and relate variations in the receding angle to the shape and movement of the three-phase contact line. It is suggested that during the receding motion the meniscus successively dewets from one post at a time, with a step function running along the contact line until it has receded from a row of posts over its entire length.

A dynamic Cassie–Baxter model

“…(the ratio of the grating width to pitch) of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8 were prepared and tested. Overall, the receding contact line periodically pinned on, slid on, and jumped across microstructures, presenting stick-slip motions consistent with observations from other researchers [8][9][10][11] . However, we were able to obtain far more details about the motion of the receding meniscus that would not have been possible without the 2-D condition and high-speed imaging.…”

Driving Droplet by Scale Effect on Microstructured Hydrophobic Surfaces

“…What's more, the maximum value of L  and R  at a certain time can be treated as advancing contact angles of the rear and the front contact lines. According to some previous researches, 21 advancing contact angle could be very large and have no relationship with the geometrical parameters of the substrate. In Figure 7, most of L  and R  are close to each other, and larger than 165 , and we could not distinguish their difference, our result for the advancing contact angles are consistent with Dorrer's 21 and Zhang's 22 result.…”

“…18 Our research demonstrates that scale is very important and should not be ignored especially in small scale. [19][20][21][22][23] The model including line tension not only can be applied to explain these phenomena, but also gives new insights to the wetting theory.…”

Design of a robust superhydrophobic surface: thermodynamic and kinetic analysis

“…Penetration depth depends on the manner, in which a surface and drop come in mutual contact 12,13 . Many of the published results on wetting experiments involve deposition of water droplets from the top [13][14][15][16][17][18] . Here, the LA interface comes into contact with the apex of the surface roughness (Cassie state).…”