Methodology for High Accuracy Contact Angle Measurement

Abstract

A new version of axisymmetric drop shape analysis (ADSA) called ADSA-NA (ADSA-no apex) was developed for measuring interfacial properties for drop configurations without an apex. ADSA-NA facilitates contact angle measurements on drops with a capillary protruding into the drop. Thus a much simpler experimental setup, not involving formation of a complete drop from below through a hole in the test surface, may be used. The contact angles of long-chained alkanes on a commercial fluoropolymer, Teflon AF 1600, were measured using the new method. A new numerical scheme was incorporated into the image processing to improve the location of the contact points of the liquid meniscus with the solid substrate to subpixel resolution. The images acquired in the experiments were also analyzed by a different drop shape technique called theoretical image fitting analysis-axisymmetric interfaces (TIFA-AI). The results were compared with literature values obtained by means of the standard ADSA for sessile drops with the apex. Comparison of the results from ADSA-NA with those from TIFA-AI and ADSA reveals that, with different numerical strategies and experimental setups, contact angles can be measured with an accuracy of less than 0.2 degrees. Contact angles and surface tensions measured from drops with no apex, i.e., by means of ADSA-NA and TIFA-AI, were considerably less scattered than those from complete drops with apex. ADSA-NA was also used to explore sources of improvement in contact angle resolution. It was found that using an accurate value of surface tension as an input enhances the accuracy of contact angle measurements.

Statistical Contact Angle Analyses with the High-Precision Drop Shape Analysis (HPDSA) Approach: Basic Principles and Applications

“…Whereas generally axisymmetric drop shapes are uncomplicated, the reproducible characterization of non-axisymmetric drop shapes is much more complicated. The famous ADSA technique [58][59][60] and its various versions is the method of choice for axisymmetric drop shapes (e.g., pendant drop) but is due to its nature not applicable to non-axisymmetric drop shapes. Unfortunately, every real surface tents to lead to non-axisymmetric drops even without inclining the sample.…”

“…Beside the Young-Laplace or Bashford-Adams fitting, common software fit one circle/ellipse or polynomial functions (tangential) onto the macroscopic drop shape. These approaches work quite well for static and axisymmetric droplets, e.g., by axisymmetric drop shape analysis ADSA [58][59][60] in its various versions but fail if strongly non-axisymmetric droplets during contact angle measurements are evaluated. In addition, pixel resolutions and the generally performed discretization of the images affect the ADSA procedure [61].…”

Accuracy of surface tension measurement from drop shapes: The role of image analysis

“…4). Recently, a different sub-pixel resolution procedure using a sigmoid was also explored [32,33]. In the sigmoid procedure, for every pixel of the drop profile, previously determined by an edge detection algorithm, a sigmoid function (g(x) in Eq.…”

“…The parameter x 0 is considered as the new location of the drop interface. It is noted that because of the multi-variable optimization, the sigmoid procedure requires much more computational time than the spline procedure, and was not pursued for this reason previously [33].…”

Laplacian drop shapes and effect of random perturbations on accuracy of surface tension measurement for different drop constellations

“…This is not an academic point. From the authors' work on contact angles [45,46], we know that the calculated surface tension is frequently erroneous. The progress to be made will have to be based on the comparison between the analysis of experimental, theoretical and distorted theoretical profiles.…”