Coatability, purity, adhesiveness: Many industrially relevant surface properties are directly or indirectly linked to wettability, which is therefore often tested as part of quality assurance. However, dewetting, i.e. the contraction and possible run-off of the liquid from the material, has rarely been investigated to date. This is particularly informative for many materials, such as hydrophobic and self-cleaning surfaces. Dewettability also reacts sensitively to material pre-treatment and is therefore an ideal test criterion. The relevant result for dewetting is the so-called retraction angle, a variant of the contact angle. The Hamburg-based measuring device manufacturer KRÜSS has now developed the Stood-up Drop for the optical measurement of this angle.
This new unit places a drop of water on the sample with high, controlled dynamics. The liquid initially spreads out flat and then contracts at lightning speed to form a defined drop. A camera captures the drop in a video image, which is analyzed to determine the retraction contact angle. The measured value is available in a matter of seconds. The speed of measurement means that the retraction angle, which was previously difficult to measure, can now be used for quality tests, even with large sample volumes.
The measurement of the retraction angle has the potential to supplement or even replace other test methods for surfaces. In extensive preliminary investigations, the measured value showed good correlations with pre-treatment parameters, such as for corona, plasma and flame treatments, but also with the results of common test methods. In some applications, prescreening with the retraction angle has already reduced the number of laborious material tests.
