The surface tension (SFT) can now also be measured using a single sessile drop. Krüss GmbH, Hamburg/Germany, is expanding its portfolio of optical interface analysis with the Constrained Sessile Drop (Constrained SD) method.
Rapid purity checks of contact angle test liquids and analyses of molten materials at high temperatures are the main areas of application.
The method observes how the opposing forces of SFT and gravity affect the shape of a droplet. If the density and dimensions of a dosed sessile drop are known, the SFT can be calculated by image analysis of its contour. For this purpose, the drop must be as large as possible, with a high curvature, while maintaining absolute symmetry. Both are achieved by placing the drop onto a perfectly circular pedestal, whose edges form a wetting barrier.
The strengths of this method lie primarily in the analysis of molten materials at high temperatures, for example hot melts whose SFT determines how they wet substrates prior to bonding. Since a sample can be melted directly on the sample pedestal, no vessels or high-temperature dosing units are required. This, in addition to the small sample volume, prevent the need for time-consuming preparation and cleaning when using Constrained SD.