Direct measurement of macro contact angles through atomic force microscopy

The information of liquid film profile near a three-phase contact line is critical for a comprehensive understanding of various wetting and phase-change phenomena. Despite numerous theoretical and simulation studies, an accurate measurement on the thin film profile is difficult due to its very small scale. In the present work, a state-of-the-art atomic force microscopy (AFM) under tapping mode (TM) was employed to achieve a high-power scanning across the contact line. Within a scale of several to tens of microns, a highly linear film profile is observed near the contact line, based on which the contact angle is extracted. Comparing to the macro contact angle measured by the traditional optical method, the AFM result shows good agreement but achieves much higher precision. Moreover, the sub-micron thin film that is beyond the capability of the optical method was observed, in which the film profile is not linear and the concept of macro contact angle is not valid.