The role of viscous and capillary forces in the prediction of critical conditions defining super-hydrophobic and hydrophilic characteristics

The prediction of the superhydrophobic/superhydrophilic behavior from the characteristics of post impinged droplet on a surface with the alteration of physical properties of fluid or the surface properties has gained much interest for the development of anti-fogging glass, gradient surface and anti-corrosion surfaces. The abovementioned characteristics are well described by the spreading, recoiling, rolling, and bouncing behaviors of the post impinged droplet and these phenomena are controlled by viscous and capillary forces. The proper combination of the capillary and viscous forces of a fluid leads to various levels of recoiling and spreading. However, in terms of the discussed forces, the conditions to attain superhydrophobicity/superhydrophilicity are not reported. Therefore, in the current work, an attempt has been made to predict critical conditions for the attainment of both superhydrophobic and superhydrophilic characteristics in terms of fluid and surface properties defining the post impingement behavior. The analysis of droplet dynamics reveals that the ratio of viscous force to capillary force (V F /C F ) is considered as the defining parameters for super-hydrophobic and hydrophilic characteristics. For the attainment of superhydrophilic characteristics, V F /C F ratio must be greater than 30 × 10 −4 and value below 2.9 × 10 −4 generates superhydrophobicity. Furthermore, the role of viscosity and surface tension in droplet dynamics were also investigated by using Central Composite Design (CCD) analysis methodology and it confirms the existence of strong interaction between the viscosity and surface tension.