Replacing the solid needle by a liquid one when measuring static and advancing contact angles

Abstract

In general, the optical determination of static and advancing contact angle is made on drops applied or extended, respectively, onto a substrate through the use of thin solid needles. Although this method is used extensively, this way of dosing can be time consuming, cumbersome and if not performed meticulously can lead to erroneous results. Herein, we present an alternative way of applying drops onto substrates using a liquid jet produced by a liquid pressure dosing system acting as a “liquid needle”. We performed a comparative static contact angle study on 14 different surfaces with two different liquids (water and diiodomethane) utilizing two different ways of dosing: the conventional solid and a novel liquid needle-based technique. We found, for all but one sample, that the obtained results on μl size drops were comparable within the experimental error bars provided the liquid needle is thin enough. Observed differences are explained by the special characteristics of either way of dosing. In addition, we demonstrate how the liquid pressure-based dosing system facilitates high-speed optical advancing contact angle measurement by expanding a drop from 0.1 to 22 μl within less than 1.2 s but yet providing constant contact angle versus drop base diameter curves. The obtained results were compared with data from tensiometric dynamic Wilhelmy contact angle measurements. These data, in conjunction with sequences of live images of the dosing process of the liquid pressure dosing system, illustrate how this system can replace the solid needle by a liquid needle.