A method for measuring contact angle of asymmetric and symmetric drops

Depending on the surface texture and external force orientations, a drop on a surface may be symmetric or asymmetric. The state-of-the-art contact angle measurement methods applicable to axisymmetric (symmetric) drops need a few of liquid parameters. Methods applicable to asymmetric drops (e.g. drops on inclined surfaces) may not be very accurate, and suitable for automated computer implementation. This paper presents a new image processing based method, i.e. sub-pixel polynomial fitting (SPPF), to measure the contact angle of symmetric and asymmetric drops. Contact angle in this method is measured by analyzing the side view image of the drop without needing any liquid parameters such as volume or density. The contact angle measurement procedure is as follows: finding the sub-pixel location of the drop boundary, detecting the exact location of the contact points, fitting a polynomial to the drop boundary near contact line and calculating the contact angle. The procedure of finding the sub-pixel location of the boundary is verified using pin gages with known diameters. Contact angle measurement algorithm is verified with the ramé-hart instrument Co. drop calibration reference tool (fixed sessile drops at nominal contact angles of 30°, 60°, 90°, and 120°) and synthetic drops with contact angles of 10–170°; less than 1° error were observed. The SPPF method is also compared with FTÅ, ADSA, and ImageTool for a wide range of contact angle of symmetric drops. The difference of measured contact angles using SPPF was less than 3°, when compared with the mean value of the FTÅ, ADSA, and ImageTool values. This error was slightly lower than the error of other methods. Short processing time, usability for all ranges of contact angles, applicability to both symmetric and asymmetric drops and being fully automatic are other advantages of the SPPF.

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► A method is developed to measure the contact angle of symmetric and asymmetric drops.
► Contact angle is measured from side view image of a drop without needing any liquid parameters.
► The error for the method is in the order of 1° over the contact angle range of 10° and 170°.