OBSERVATIONS OF SOLUTE EFFECTS ON BUBBLE FORMATION

We have studied the effects of solute, in particular aqueous electroly te, on bubble formation at capillary orifices (diameters from 50 mu m to 1 mm) and frits at varying gas flow rates. Using a stroboscope, vid eo microscope, and rotating mirror, we have obtained pictures which sh ow how bubble formation involves the interaction of bubbles at the ori fice. These interactions depend on the value of the surface elasticity E (proportional to c(d gamma/dc)(2)) due to positively (e.g,, ethanol ) or negatively (e.g., NaCl) adsorbed solute. At low flow rates consec utive bubbles do not interact. Each bubble detaches and leaves the ori fice region before the next one starts forming. At intermediate flow r ates the more closely spaced, consecutive bubbles begin to interact. I n pure liquids there is no barrier to bubble coalescence and the detac hed 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 co alescence is inhibited. Instead of ''feeding'' into the detached bubbl e the following bubble pushes it aside, and the bubbles appear to ''bo unce'' off each other. Bouncing may give rise to a characteristic sequ ence of larger and smaller bubbles (often as sidestreams) if the emerg ing bubbles break off prematurely from the orifice due to the inertia of the original bubble. The transition from feeding to bouncing depend s critically on E of the solution and leads to a smaller average bubbl e size for larger E values. At high flow rates detached bubbles are in variably fed by several subsequent ones, regardless of the surface ela sticity. At very high flow rates the bubbling becomes chaotic, but the interaction of bubbles after leaving the orifice area produces smalle r bubbles in solutions. In general, bouncing is more likely to occur w ith narrow and irregular capillaries. The dramatically different appea rance of gas-sparged columns in salt water and freshwater has its orig in in the differences between assemblies of pores showing mainly feedi ng (freshwater) or bouncing (saltwater). (C) 1995 Academic Press, Inc.