(246f) Collisions and Coalescence of Micro-Bubbles Measured Using Atomic Force Microscopy

The dynamic forces between micro-bubbles are crucial for understanding applications as wide ranging as foams in mineral processing to ultrasound contract agents. We employ gas bubbles in water, the simplest and arguably the most shape sensitive soft matter systems, to quantitatively link the dynamic coupling of bubble shape with external forces that control their behavior. We have extended the experimental and theoretical methods developed to study the dynamic interactions between droplets using Atomic Force Microscopy (AFM) or the interactions between a micro-bubble and a flat surface in aqueous solution to study the collisions and coalescence behavior of two micro-bubbles in aqueous solution. These measurements show that bubble collisions, even at speeds comparable to Brownian motion, are dependent on a combination of equilibrium surface forces, hydrodynamic drainage forces and interfacial deformation. Quantitative modeling, which shows excellent agreement between experiments and calculations, demonstrates that the variations in the dynamic forces arise from changes in both the equilibrium surface forces and hydrodynamic drainage effects. These results have implications in applications as wide ranging as froth floatation to micro-fluidics.