A new insight into dynamic contact angle measurements: Implementation of smooth splines method

The contact angle (CA) of a drop is a common quantitative measurement that describes a surface's wettability. However, manual selection of the drop's solid-liquid-vapor contact points (CPs) by the user introduces substantial human error to the final measured CA. Hence, in this article we introduce a novel method for CA measurement that uses a smoothing spline function to simultaneously cancel probable image noise, find the solid-liquid contact point, and calculate the CA at the contact point. The accuracy of this technique is tested through measurement of a series of synthetic drop images with known CAs ranging from 5 degrees to 175 degrees,which are generated using a new mathematical approach and include a reflection area to accurately model real experiments. In these tests, the smoothing splines approach is shown to be extremely accurate in measuring both the CP position and the CA, with negligible error (R-2 = 0.9999). Then, for added validation, the smoothing splines approach is tested on four real surfaces of different wettability: glass, single crystal silicon, polished polytetrafluoroethylene, and patterned superhydrophobic polytetrafluoroethylene. When tested on real surfaces, the smoothing splines measurement approach provides reliable contact angle data for all four different surfaces tested, while also reducing human error. Overall, our proposed measurement approach for the CA is shown to be more accurate than traditional measurement approaches such as the commonly used polynomial fitting approach, and the more recently introduced mask approach, which also includes automatic CP detection.