Spreading dynamics of water onto soluble polymer coatings with hydrophobic insoluble patterns

The wetting of many food powders is affected by hydrophobic ingredients present at the surface, such as fat. Instant beverages, soups and infant formulas are typical examples.
The spreading of sessile droplets on homogeneous soluble polymer films is dominated by the hydration resulting from water evaporation and condensation ahead of the moving contact line^1,2,3, resulting in a strong dependence of the contact angle (θ) on the contact line speed (U), film thickness and local water content.
This study introduced surface heterogeneity by inkjet printing hydrophobic fat patterns at controlled area fractions 0-50% onto thin soluble polymer coatings of maltodextrin to assess the impact on the water spreading dynamics, at controlled ambient RH. Increasing the area coverage increased strongly θ, for every U. A surface coverage of 26.3% was found to halve U after 10 s of spreading. Fat patterns were shown to alter both the interface shape and the contact line length. A theory is proposed to describe the impact of these hydrophobic insoluble patterns on the water spreading dynamics.
REFERENCES:
1. Tay A. et al. Eur. Phys. J, 2010, 33, 203–210.
2. Dupas J. et al. Lagmuir, 2013, 29, 12572–12578
3. Dupas J. et al. Phys.Rev.Lett, 2014, 112, 188302