(82c) Interaction Forces in Relation to Agglomerate Size of Hydrophobic Particles

Various industrial processes deal with poor wetting solids in liquids like filtration, flotation and flocculation. These processes are highly influenced by the appropriate particle interactions. Beside the omnipresent van der Waals interaction between the hydrophobic particles in aqueous solutions further attractive forces occur, that are not described by the classical DLVO-theory. These are so called hydrophobic forces, whose origins are not completely understood yet. A common explanation for the establishment of the long range (> 100 nm) hydrophobic force is a formation of gas nanobubbles, which are cap shaped and strongly fixed on the hydrophobic particle surface. If nanobubbles-covered particles collide, it is possible to form adhesive capillary bridges. In this case, higher adhesion forces are acting in aqueous solutions due to the three phase contact and result to strong agglomeration.

The present study deals with the impact of wetting on the agglomeration performance of alumina particles in a stirred tank, filled with an aqueous solution. Alumina particles (< 25 µm) are treated with a hydrophobic silane coating that leads to a planar water-contact angle of 105°. Thereby, the hydrophobic adhesion mechanism is acting between the particles and increases the agglomerate size as well as the agglomerate strength. Furthermore the adhesive force distributions were measured by atomic force microscopy in spectroscopy mode between a colloidal probe and a rough alumina surface as well as alumina particles. Additionally, using AFM at intermittent mode, nanobubbles were detected on representative alumina samples in the same physico-chemical environment as used in the agglomeration experiments.