Bubble coalescence in electrolytes: effect of bubble approach velocity

Abstract The coalescence of air bubbles in aqueous solutions of electrolytes was studied experimentally. The bubbles were produced from two vertical parallel capillaries in a still liquid and video recorded. Several quantities were evaluated: bubble shape and size, bubble distance, bubble expansion rate, time of the first touch, time of bubble merger, time of bubble detachment. The three state variables were: bubble coalescence efficiency E, transition electrolyte concentration Ct, bubble contact time - coalescence time T. The effect of four control parameters was investigated: electrolyte type (3 inorganic salts: NaCl, Na2SO4, CaCl2), electrolyte concentration C (5 values sparsely covering a broad interval, 0.001 - 2 mol/L), bubble approach velocity V (9 values, from 0.03 to 40 mm/s), bubble diameter D (2 values, 1 and 1.5 mm). The following results were obtained. Coalescence efficiency E decreases with both C and V. Transition concentration Ct is not a material property but depends on the hydrodynamic conditions, Ct decreases with V. Contact time T decreases with V, as T ∼ V-1, and increases with C, roughly as T ∼ C1.9. Bubble approach velocity V plays a dual role: it reduces the coalescence region but accelerates the coalescence process. The transition from coalescence to noncoalescence proceeds via a minimum in the T(V) graphs. Bubble size D reduces the value of Ct, whence the extent of the coalescence region. The three electrolytes behave in a similar way. A scale-estimates model was developed to describe the results. The effect of electrolytes on the coalescence is similar to the effect of liquid viscosity.