Rolling, penetration and evaporation of alcohol–water drops on coarse and fine hydrophobic powders

Agglomeration of hydrophobic powders using aqueous solutions is an important technology. The outcome is linked to the particle wettability, but the mechanisms are not well-understood. We present a fundamental study of the early stages of agglomeration. Drops of alcohol–water mixtures were deposited onto three hydrophobic powders: hydrophobised glass beads (spherical, 90–106 μm), coal dust (angular shape, 90–106 μm) and molybdenite powder (angular shape, 1–20 μm). Their wettability was manipulated through the surface tension of the liquid, which was varied by the alcohol–water ratio. On coarse powders (glass beads and coal dust) the droplet either penetrates into the porous bed (at low surface tensions and contact angles), or sits on top and evaporates (at higher surface tensions and contact angles). Particles spontaneously coat drops of intermediate surface tension. The coating slows evaporation and remains as a dry agglomerate after evaporation. Drops that do not penetrate into the fine powder beds (molybdenite) spontaneously roll away, even when deposited carefully. The reason is that the powder surface behaves like a superhydrophobic one. Rolling drops collect hydrophobic particles along their path. After rolling, the partially coated drop evaporates in the same way as on coarser powders.