Measurements of interactions between particles and charged microbubbles using a combined micro- and macroscopic strategy.

We report the use of atomic force microscopy (AFM) to study the interactions between silica glass colloidal probes and charged microbubbles created using one of two different surfactants: anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethylammonium bromide (DTAB) in an aqueous environment. On close approach between the glass probe and a SDS microbubble, an appreciable repulsive force was observed prior to contact. This was not observed when using a DTAB microbubble, where only attractive forces were observed prior to contact. zeta-potential analysis showed that silica surfaces are negatively charged across the pH range of 3-10 when surfactant is not present. Addition of SDS did not alter the zeta-potential significantly, indicating that adsorption onto the particle surface did not occur. Conversely, the addition of DTAB decreased the negativity of the zeta-potential, reversing the sign, indicating that adsorption had occurred. This analysis was used in the removal of fine particles from suspension using charged microbubbles. Silica particles were recovered using positively charged microbubbles from DTAB but not when using negatively charged microbubbles generated from SDS. Taken together, the data suggest that repulsive long-range interactions were responsible for the selective attachment of silica particles to microbubbles in a charge-dependent manner.