Adaptation of application of drop area method to characterize hydrophobicity of materials

Abstract

The drop area can characterize hydrophobicity of materials. To improve accuracy of the drop area method, a large number of drop profiles with different contact angles, drop volumes and liquid parameters (Δρ/γ) are numerically generated. Based on the generated profiles, the influences of contact angle, drop volume and liquid parameter on the drop area and its sensitivity dS/dθ are systematically investigated. The results reveal that the sensitivity of the drop area method reduces with increasing contact angle, and increases with increasing drop volume and liquid parameter. The drop area method is particularly suitable for application in the case with small contact angle and at the same time, the drop volume and liquid parameter should be not so low. The relationship between water drop area and contact angle at some common drop volumes is obtained by numerical calculation. The concept of critical contact angle for the drop area method is proposed and the values of critical contact angle corresponding to different water drop volumes are presented. And it can be used to determine whether the drop area method or the static contact angle method should be selected. Finally, the contact angle method and the drop area method are used to measure hydrophobicity of glass and silicone rubber with different water drop volumes. The results validate the above analysis and at the same time it is consistent with the corresponding numerical calculation. This work is helpful to improve the accuracy in the drop area method and to generalize the use of it.