Indexed on: 06 Sep '18Published on: 06 Sep '18Published in: Langmuir
The coalescence between microbubbles and millimeter size bubbles is an elementary process in various industrial applications such as froth flotation and wastewater treatment. Fundamental understanding of the coalescence behavior between two colliding bubbles requires knowledge of water drainage from the thin liquid film between the deformable air-water surfaces, a simple phenomenon with high complexity in physics due to the interplay of surface forces, hydrodynamic drainage, and surface rheology. In this work, we performed simultaneous measurements of the interaction force and spatial thin film thickness during the collision between a millimeter size bubble (radius 1.2 mm) and surface microbubbles (radii between 30 to 700 µm) using our recently developed dynamic force apparatus (DFA). The interaction force during the collision agrees well with the prediction from the Stokes-Reynolds-Young-Laplace model with the tangentially immobile boundary condition at the air-liquid interface. However, the measured coalescence times for different bubble sizes are shorter than the model predictions, possibly caused by a rapid drainage behavior along with the loss of symmetry of the thin liquid film. In dozens of experimental runs, the bubbles coalesced at a critical film thickness of 25±15 nm, which agrees reasonably well with the predicted rupture thickness using attractive van der Waals interaction force. These results suggest that the non-symmetric drainage process, rather than the rupture thickness, contributes to the scattering of the experimental coalescence time between two fast colliding air bubbles. Furthermore, our results suggest that smaller surface bubbles (30 - 100 µm) are more effective for the attachment onto a large bubble as the coalescence time decreases considerably when the microbubbles are smaller than 100 µm.