Simultaneous Measurement of Contact Angle and Surface Tension Using Axisymmetric Drop-Shape Analysis-No Apex (ADSA-NA)

Abstract

Axisymmetric drop-shape analysis-no apex (ADSA-NA) is a recent drop-shape method that allows the simultaneous measurement of contact angles and surface tensions of drop configurations without an apex (i.e., a sessile drop with a capillary protruding into the drop). Although ADSA-NA significantly enhanced the accuracy of contact angle and surface tension measurements compared to that of original ADSA using a drop with an apex, it is still not as accurate as a surface tension measurement using a pendant drop suspended from a holder. In this article, the computational and experimental aspects of ADSA-NA were scrutinized to improve the accuracy of the simultaneous measurement of surface tensions and contact angles. It was found that the results are relatively insensitive to different optimization methods and edge detectors. The precision of contact angle measurement was enhanced by improving the location of the contact points of the liquid meniscus with the solid substrate to subpixel resolution. To optimize the experimental design, the capillary was replaced with an inverted sharp-edged pedestal, or holder, to control the drop height and to ensure the axisymmetry of the drops. It was shown that the drop height is the most important experimental parameter affecting the accuracy of the surface tension measurement, and larger drop heights yield lower surface tension errors. It is suggested that a minimum nondimensional drop height (drop height divided by capillary length) of 1.7 is required to reach an error of less than 0.2 mJ/m(2) for the measured surface tension. As an example, the surface tension of water was measured to be 72.46 ± 0.04 at 24 °C by ADSA-NA, compared to 72.39 ± 0.01 mJ/m(2) obtained with pendant drop experiments.

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

“…The numerical integration block has been improved to its utmost limit [12], and the optimization procedure was shown [20] and also will be shown again here to have no effect on the results. Therefore only edge detection needs to be pursued and perfected here.…”

“…Zuo et al also implemented a method called component labeling to remove noise from the image [27]. Kalantarian et al [20] introduced a nongradient edge detector called SUSAN [28] into a new version of ADSA. It was also claimed [28] that SUSAN is more robust to noise in the image than gradient edge detectors (e.g.…”

“…Canny and Sobel). To test these matters further, the results obtained by SUSAN were compared more recently [20] with the results obtained by three different gradient edge detectors (i.e., Canny, Sobel and Laplacian of Gaussian (LoG)) [29]). It was found that there is more consistency among the results obtained with Canny, Sobel, and SUSAN than for those obtained with LoG.…”

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].…”