Observations of Solute Effects on Bubble Formation

Abstract

We have studied the effects of solute, in particular aqueous electrolyte, on bubble formation at capillary orifices (diameters from 50 μm to 1 mm) and frits at varying gas flow rates. Using a stroboscope, video microscope, and rotating mirror, we have obtained pictures which show how bubble formation involves the interaction of bubbles at the orifice. These interactions depend on the value of the surface elasticity E (proportional to c (dγ/dc)2) due to positively (e.g., ethanol) or negatively (e.g., NaCl ) adsorbed solute. At low flow rates consecutive bubbles do not interact. Each bubble detaches and leaves the orifice region before the next one starts forming. At intermediate flow rates the more closely spaced, consecutive bubbles begin to interact. In pure liquids there is no barrier to bubble coalescence and the detached bubble is "fed" by the subsequent bubble as this starts to grow. The process may be repeated several times before the original bubble has risen out of range. In solutions where E is large enough bubble coalescence is inhibited. Instead of "feeding" into the detached bubble the following bubble pushes it aside, and the bubbles appear to "bounce" off each other. Bouncing may give rise to a characteristic sequence of larger and smaller bubbles (often as sidestreams) if the emerging bubbles break off prematurely from the orifice due to the inertia of the original bubble. The transition from feeding to bouncing depends critically on E of the solution and leads to a smaller average bubble size for larger E values. At high flow rates detached bubbles are invariably fed by several subsequent ones, regardless of the surface elasticity. At very high flow rates the bubbling becomes chaotic, but the interaction of bubbles after leaving the orifice area produces smaller bubbles in solutions. In general, bouncing is more likely to occur with narrow and irregular capillaries. The dramatically different appearance of gas-sparged columns in salt water and freshwater has its origin in the differences between assemblies of pores showing mainly feeding (freshwater) or bouncing (salt water).