Article Cite This: Langmuir XXXX, XXX, XXX−XXX
pubs.acs.org/Langmuir
Contact Angle Determination on Hydrophilic and Superhydrophilic Surfaces by Using r−θ-Type Capillary Bridges Norbert Nagy* Institute of Technical Physics and Materials Science, HAS Centre for Energy Research, P.O. Box 49, H-1525 Budapest, Hungary
Downloaded via UNIV AUTONOMA DE COAHUILA on April 4, 2019 at 05:59:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: To avoid the restrictions of the captive bubble and the Wilhelmy plate techniques, a method was introduced for contact angle measurements under equilibrium conditions. It enables to determine even ultralow contact angles with high precision without prewetting the investigated surface because in this case, the capillary bridge of the test liquid is formed from a pendant drop and used as a probe. The contact angle is determined from the measured capillary force and liquid bridge geometry by using Delaunay’s analytical solution. The method was experimentally proved to be valid. As a demonstration, contact angles less than 1° were measured with the uncertainty down to 0.1° on lightly corroded glass surfaces. Moreover, a new observation was obtained in complete wetting situations: the receding contact line starts to advance again during the increase of the bridge length. The contact angle is much lower in this readvancing phase compared to the advancing and receding values because the contact line finds prewetted surface in front of itself. Further advantage of the method is that the existing contact angle goniometers can be developed further into the presented measurement setup. problem can be the captive bubble method,5 which inherently fulfills the requirement of saturated vapor. The drawback of this method is that the needle usually must be left in the bubble to hold it in the right place; therefore, the analysis of the bubble’s shape is less accurate. Furthermore, the characterization of certain surfaces, e.g., swelling solids, coatings, and films, is problematic due to the immersion. The sessile drop and the captive bubble methods show drop size effect6 also on smooth and heterogeneous surfaces; however, the relationship between the contact angle and the drop size is affected by the scale of roughness or heterogeneity.2 The Wilhelmy balance method7,8 is another good solution to determine low contact angles. It has high sensitivity and accuracy due to the measurement of wetting force, and it enables to study adsorption processes.9 There are important requirements to reach this accuracy: the length of the contact line should be known with high precision, i.e., the sample geometry is restricted; furthermore, all surfaces along the contact line must be identical. In general, the characterization of films, coatings, and anisotropic surfaces by the Wilhelmy method is difficult or not possible. Another technique uses a simple apparatus to simultaneously measure the volume and the contact radius of a sessile droplet (SD). In this case, even low contact angles can be calculated using a semiempirical formula.10 Optical microscopy is also used to
1. INTRODUCTION The measurement of contact angle formed on the solid− liquid−vapor triple line is one of the most important surface characterization techniques for a long time. The measurement of the advancing and receding contact angles under static conditions on a solid surface enables the quantification of its wettability and the determination of surface free energy, and it provides information about its rough or heterogeneous character. Undoubtedly, the most popular measuring technique is the sessile drop method due to its relative simplicity. Its precision was typically ±2° in the past, which was significantly improved (
