Droplet and bubble wetting behaviors: The roles of surface wettability and roughness

Droplet and bubble wettability are important parameters for characterizing the wetting propensity of solid surface. Recent studies have shown significant differences between the wetting behaviors of droplet and bubble, specifically apparent contact angle (ACA) and contact angle hysteresis (CAH). However, very few studies have systematically investigated and examined the differences. Additionally, there is a lack of comprehensive knowledge and experimental observations about the effects of surface roughness on the droplet and bubble wetting behaviors under different wetting states. These knowledge gaps restrict our understanding of the wetting dynamic process. In this study, by varying surface roughness and modifying wettability with silane coupling agent KH-550, we evaluated droplet and bubble CAHs from strong hydrophilicity to hydrophobicity using a novel gas-driven method and the moving needle method, and analyzed the measurements using the friction force model. Young contact angle (YCA) was calculated to characterize the intrinsic wettability of three-phase system and experimental observations were interpreted in detail. The results show that the impact of roughness on droplet CAH varies in different wetting states caused by coupled effect between solid-liquid affinity and rough microstructure. A critical YCA for effective spreading is observed in bubble wetting behaviors for the first time, above which bubble ACA and CAH abruptly change. The increase of roughness reduces bubble ACA and CAH. With a critical Young contact angle as the turning point, the difference between droplet and bubble wetting behaviors as well as the effect of roughness vary. Additionally, after bubble fully spreads, its advancing angles are equal to droplet ACAs, and its ACAs are equal to droplet receding angles on relatively smooth surface. These intriguing findings are supplementary to current knowledge about droplet and bubble wetting behaviors.