Contact angles and wetting behaviour of single micron-sized particles

A ‘particle interaction apparatus’ based on the technique of atomic force microscopy was constructed that allows us to measure the interaction between single micron-sized particles and the air–water interface. From the force versus distance profiles (‘force curves’) the contact angle of single microspheres could be determined. This new method for microsphere tensiometry was validated using a variety of materials with contact angles between 20° and 90°. Contact angles measured on single microspheres correlated well with those measured on flat substrates of the same materials. The interaction of single silica microspheres with an air bubble in the presence of surfactants (SDS and DTAB) was investigated. Depending on surfactant type and concentration, adhesion or repulsion could be induced. Adhesion forces were found to depend on the applied load, indicating possible adsorption/desorption processes at the particle–bubble interface. We have built a new set-up that combines a particle interaction apparatus with a Langmuir trough and a fluorescence microscope. This will allow study of interactions at the air–water interface in more detail, especially in the presence of a definite surface density of amphiphilic molecules. The interaction of single ZnS spheres with a bubble (modelling flotation of ZnS) was studied at different pH values. The results suggest that the isoelectric point of these spheres exists between pH 7 and 8.

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