Automation of the axisymmetric drop shape analysis-diameter for contact angle measurements

Abstract

Axisymmetric drop shape analysis-diameter (ADSA-D) was developed to measure contact angles on non-ideal and/or hydrophilic surfaces (e.g. biological surfaces). ADSA-D determines the contact angle by solving the Laplace equation of capillarity numerically, and using as input the liquid surface tension, the drop volume and diameter of the drop as viewed from above. The diameter is the main experimental parameter to be determined as the liquid surface tension is known and the volume of the sessile drop can be obtained precisely by means of the micrometer syringe used to form the drop. The edge detection method to determine the diameter, which consisted of manually extracting coordinates from a digital image was found to be time consuming and somewhat subjective. Therefore, an automated image processing module was developed to replace the manual scheme. This automated module uses nonlinear filters to process the images and uses a region growing scheme to determine the total area of the drop as viewed from above. This area is then used to calculate the equivalent diameter of the idealized contact or equatorial circle for the drop. The robust design of the automated module enables it to process accurately many different types of drop images. Two very different examples of water drops on bacteria and carboxylic acid terminated self assembled surfaces are used to demonstrate the capabilities of the new image processing scheme. Also, using these two examples, it is shown through contact angle measurements that the results of this new ADSA-D version are more accurate than those obtained through the manual digitization scheme previously used.