Coalescence or Bounce? How Surfactant Adsorption in Milliseconds Affects Bubble Collision.

The coalescence between two colliding bubbles in ultraclean water can be 3 or 4 orders of magnitude faster than coalescence in contaminated solutions. This surprising result can be mostly explained by the mobile or immobile boundary conditions at the air-water interface. In this work, we employ a rising bubble technique to study bubble collisions in aqueous solutions with up to 2 mM surfactant. The experimental results clearly show that freshly generated bubbles can coalesce within milliseconds if they collide right after generation. However, once the bubbles reside in the bulk for tens of milliseconds, the coalescence is heavily hindered. Considering these results, we conclude that a clean air-water interface, rather than clean water, is required to achieve the mobile boundary condition that allows quick coalescence. These findings provide fundamental understanding for further improvements in bubble generation that will benefit industrial processes such as mineral flotation, oil extraction, and wastewater treatment.