A modified selected-plane method for contact angle (theta) measurement is proposed in this study that avoids the difficulty of finding the real contact point and image-distortion effects adjacent to the contact point. This method is particularly suitable for superhydrophobic surfaces. The sessile-drop method coupled with the tangent line is the most popular method to find the contact angle in literature, but it entails unavoidable errors in determining the air-solid base line due to the smoothness problem and substrate tilting. In addition, the tangent-line technique requires finding the actual contact point. The measurement error due to the base line problem becomes more profound for superhydrophobic surfaces. A larger theta deviation results from a more superhydrophobic surface with a fixed base line error. The proposed modified selected-plane method requires only four data points (droplet apex, droplet height, and two interfacial loci close to the air-solid interface), avoiding the problem of the sessile-drop-tangent method in finding the contact point and saving the trouble of the sessile-drop-fitting method for best fitting of the numerous edge points with the theoretical profile. A careful error analysis was performed, and a user-friendly program was provided in this work. This method resulted in an accurate theta measurement and a method that was much improved over the classical selected plane and the sessile-drop-tangent methods. The theta difference between this method and the sessile-drop-fitting method was found to be less than three degrees.

The dynamic wetting behaviors, especially the droplet morphology, of a water droplet impinging on five substrate surfaces were investigated. A water drop was released from 13.6 mm above a solid surface and impinged on substrates. The images (the silhouette and 45 degrees top view) were sequentially recorded from the moment that the droplet impacted the solid surface until it reached equilibrium. The entire profile of each of the water droplets during spreading was obtained from the digitized recorded images. The digitized droplets were then used to detail the spreading mechanism, including information on the relaxations of the wetting diameter, droplet height, contact angle, and spreading velocity. A comparison of the full droplet profiles allows us to clarify the independent motion of two related but independent components, the central region and rim, of an impinging droplet. An interesting plateau region in the droplet height relaxation curve was observed in the first cycle for all substrate surfaces. For hydrophobic surfaces (paraffin and Teflon), three particular growth modes in the droplet height relaxation curve were detected in every oscillation cycle during the early spreading stages. It only took three and four oscillation cycles for a water droplet on the glass and quartz substrates, respectively, to dissipate its energy and reach its equilibrium state. However, it took 72 and 28 oscillation cycles for a water droplet on the Teflon and paraffin substrates, respectively. Moreover, several other new phenomena were also observed.

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