STABILITY OF FOAMS IN SILICATE MELTS

Bubble coalescence and the spontaneous disruption of high-porosity foa ms in silicate melts are the result of physical expulsion of interpore melt (syneresis) leading to bubble coalescence, and diffusive gas exc hange between bubbles. Melt expulsion can be achieved either along fil ms between pairs of bubbles, or along Plateau borders which represent the contacts between 3 or more bubbles. Theoretical evaluation of thes e mechanisms is confirmed by experimental results, enabling us to quan tify the relevant parameters and determine stable bubble size and crit ical film thickness in a foam as a function of melt viscosity, surface tension, and time. Foam stability is controlled primarily by melt vis cosity and time. Melt transport leading to coalescence of bubbles proc eeds along inter-bubble films for smaller bubbles, and along Plateau b orders for larger bubbles. Thus the average bubble size accelerates wi th time. In silicate melts, the diffusive gas expulsion out of a regio n of foam is effective only for water (and even then, only at small le ngth scales), as the diffusion Of CO2 is negligible. The results of ou r analyses are applicable to studies of vesicularity of lavas, melt de gassing, and eruption mechanisms,