As part of the quality assurance of surfaces, wettability is often investigated, but dewetting - the contraction and run-off of liquid from the material - is rarely examined. This is informative for many materials, such as hydrophobic or self-cleaning surfaces. Dewettability also reacts sensitively to material pre-treatment. It 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.
Measured value recorded in seconds
The dosing unit of the Stood-up Drop places a drop of water on the sample with high, controlled dynamics. The liquid initially spreads out flat and then contracts into a defined drop in a flash. A camera captures the drop in a video image, which is analyzed to determine the retraction contact angle. The measured value is available within seconds.
Retraction angle is easier to check
Thanks to the speed of the Stood-up Drop, the retraction angle, which was previously difficult to measure, can now be used for quality tests, even with high sample volumes. The measuring procedure and dosing parameters are predefined, which ensures user-independent results. The method does not use any harmful substances and is therefore safer than testing with test inks.
Suitable for other test methods
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. For example, prescreening with the retraction angle has already reduced the number of material tests in some applications.
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