Abstract

Water sliding angle is one of common parameters that indicates the hydrophobicity and water repellency of a material surface. Drop shape analysis technique was normally used to measure the sliding angle by video imaging a water drop rolling off the rotated surface. The typical baseline of a flat surface was determined from consecutive drop shape images, resulting the relationship between the tilt angle against the acquisition time. To apply this method for a rough surface such as paper, cloth, porous material, or non-woven fabric, an additional experimental setup was introduced in this work to reduce the difficulties in the linear regression calculation of the wavy data points. The material was attached to a flat surface reference, which was imaged and extracted to a rotated straight baseline. In addition, the imaging latency of the camera and the rotation lag time of the angular rotor were suppressed by monitoring the image brightness that was synchronized to the rotation duration. The sliding angle was then precisely calculated by using the computer algorithms written in MATLAB. It was found that our proposed experimental approach and the computational algorithms yielded such accurate and reliable water sliding angle of the rough surface, significantly different from those calculated by the conventional setup. These results suggested that our method could be useful as a handy tool to study the water repellency of advanced materials.