Computerised Measurement of Contact Angles

Abstract

The measurement of the contact angle formed by a droplet of liquid placed on a horizontal surface – the so-called sessile drop – has been of interest to scientists and others for at least 200 years, since Young first reported his observations [1]. From this parameter, much valuable information can be calculated, notably surface energy values. These in turn can provide information on surface contamination or the wettability of a surface [2]. For this reason, the measurement of contact angles is of importance in a wide range of scientific and technological fields, including medicine, surface science, surface engineering, and industries producing inks and coatings for plastics and textile goods as described by Adamson [3], Hansen [4], Zisman, and coworkers [5]. The earliest measurements, such as that of Young, used a protractor or a similar graduated scale for measuring the angle. Various other techniques were developed, such as the so-called half-angle method, discussed below. The assumption that the sessile drop was spherical, or formed part of a sphere, underpinned the basis of these methods wherein the contact angle values were computed using the principles of Euclidian geometry. The two most widely-used such methods were: – Constructing a tangent by drawing a line orthogonal to the drop radius that intersects the point of contact with the horizontal surface – the triphase point; – The so-called half-angle method uses a line drawn from the triphase point to the apex of the circle (Fig. 1). This is of course valid only for perfect circles. Over the years, there have been modest advances, notably US Patent 5,268,733 where an image of the drop is projected onto a protractor screen [6]. Rather than being calibrated in degrees, the screen is calibrated at half-scale. The protractor can be moved to the triphase point, and the trace that intersects the apex will give the contact angle. This approach is inherently imprecise since the apex is a flat region covering a range of angles. There have also been several specialized advances customized for production-line environments [7]. Computerised Measurement of Contact Angles By Darren L. Williamsa, Anselm T. Kuhnb, Mark A. Amanna, Madison B. Hausingera, Megan M. Konarika and Elizabeth I. Nesselrodea