On the effect of insoluble hydrophobic heterogeneities on the wetting dynamics of soluble thin films

Abstract

Recent studies have highlighted the complex mechanisms governing the spreading of a solvent onto a homogeneous soluble film, such as water on soluble polysaccharides. The presence of surface fat slows down the wetting of spray dried food powders, but this phenomenon is not yet understood quantitatively. In this study, surface heterogeneities were created by the ink-jet printing of cocoa butter onto water-soluble maltodextrin thin films to produce hydrophobic deposits with a range of area coverages. The spreading dynamics of water was studied controlling relative humidity. Area coverages above 1.2 % were found to decrease the contact line speed and increase the contact angle. The contact line was deformed by the deposits of cocoa butter, causing in some cases periodic decelerations followed by accelerations. Area coverages above 26 % led to a three-fold increase in the spreading time. These novel insights could help to design soluble heterogeneous surfaces meeting a desired wetting performance.