Effect of heterogeneous hydrophobic coating on floating of insoluble particles

Food powders often exhibit hydrophobic surface areas. To understand the effect of corresponding wettability variations on particle floating, which critically influences powder reconstitution, we studied floating of glass beads after creating surface heterogeneity by spraying cocoa butter. Force-position curves were recorded to characterise contact line pinning.

Floating of particles with surface heterogeneities comprised a dynamic phase, which to our knowledge has not been reported previously. Measured forces varied due to contact line pinning/depinning. The results indicate that the dynamic floating phase is caused by gradual depinning. Steady-state floating is reached once pinning is sufficiently pronounced.

We also demonstrate that surface heterogeneities complicate theoretical predictions of particle floating. The use of sessile drop contact angles for modelling was identified as an important source of error, as weak pinning points can affect contact angles but are oftentimes overcome during floating. Moreover, local pinning effects are not necessarily represented by a sessile drop.