Prediction of Droplet Sliding on the Continuity of the Three-Phase Contact Line

Abstract

Many animals and plants have evolved wonderful hydrophobic abilities to adapt to the complex climate environment. The microstructure design of a superhydrophobic surface focuses on bionics and will be restricted by processing technology. Although certain functions can be achieved, there is a lack of unified conclusion on the wetting mechanism and a few quantitative analyses of the continuity of the three-phase contact line. Therefore, the relationship between the surface microstructure of the lattice pattern and the critical sliding angle of the water droplet in the Cassie state was investigated in this paper, and we proposed a method to quantitatively analyze the continuity of the three-phase contact line by a dimensionless length f. The results showed that the three-phase contact line was an important factor to determine the sliding performance of the droplet. The upward traction force generated by the surface tension through the force analysis on the three-phase contact line can enhance the sliding ability of the droplet on the solid surface. There was a good negative linear correlation between the critical sliding angle and dimensionless length, which provided a guiding basis for the optimal design of superhydrophobic surfaces.

Slippery Mechanism for Enhancing Separation and Anti-fouling of the Superhydrophobic Membrane in a Water-in-Oil Emulsion: Evaluating Water Adhesion of the Membrane Surface

“…The larger the section height h of the pit, the more prone it is to generate a huge pressure difference P at the gas−liquid interface; this will cause the water droplet surface in the pit to bend and a very small depth of the water droplet to immerse in the pit, resulting in a large amount of air being trapped in the rough structure. 23,53 Consequently, the degree of "suspension" of the water droplets on the microstructure increased, and the water droplets slid more easily on such a surface; thus, the final membrane surface showed excellent WCA of more than 150°a nd a sliding performance. 45,47,54 This explains well the variation trend of the WCA and SA in Figure 5a.…”

“…However, the periodic distance L and diameter d of the micropapillae differed very little from those of (M-6); thus, the water droplets still penetrated easily into the pits but not into the submicro-scale particle structure. 36 The P generated could mitigate the degree of penetration but was very limited; 23,53 thus, the WCA of (M-5) increased, yet the water droplets still had difficulty sliding on its surface, exhibiting high water adhesion. The periodic distance of the micro-scale structure is therefore also an important factor in the hydrophobicity and sliding performance.…”

“…55−58 In addition, recent studies have reported that sharpening the diameter of the micropapillae is conducive to reducing water adhesion. 59 This is because a reduction in the periodic distance and diameter of the micropapillae allowed more air to be trapped to enlarge the P 23,53 as well as reduce the area of direct contact between the water droplets and the micropapillae, 60 as shown in Figure 5b3. Therefore, the diameter of the micro-scale structure should also be taken into account.…”

“…In conjunction with SEM image analysis, the hydrophobicity and sliding performance were impacted by the relationship between the pressure difference (between the pressure at the bottom of the micropapillae and the internal pressure of the liquid) at the gas–liquid interface, and here, the section between adjacent micropapillae could be seen as a pit, as shown in Figure b1. The larger the section height h of the pit, the more prone it is to generate a huge pressure difference P at the gas–liquid interface; this will cause the water droplet surface in the pit to bend and a very small depth of the water droplet to immerse in the pit, resulting in a large amount of air being trapped in the rough structure. , Consequently, the degree of “suspension” of the water droplets on the microstructure increased, and the water droplets slid more easily on such a surface; thus, the final membrane surface showed excellent WCA of more than 150° and a sliding performance. ,, This explains well the variation trend of the WCA and SA in Figure a.…”

“…due to their outstanding purification efficiency and water-repellent behavior. − The combination of hierarchical micro-/nano-structures and low surface energy materials is recognized as an effective method for constructing superhydrophobic membrane surfaces. ,,− Table S1 gives some typical examples of superhydrophobic (and highly hydrophobic) microporous membranes prepared in recent years. As seen in Table S1, numerous studies have reported the successful preparation of membrane surfaces with a high water contact angle (WCA), yet only part of these studies involved insights into the sliding angle (SA); ,,− besides, some membrane surfaces exhibited adhesive (i.e., high water adhesion) characteristics in oil–water separation applications. , …”