Mechanisms of Foam Generation in Glass-Bead Packs

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Abstract
Summary: The fundamental, pore-level mechanisms of foam generation are investigated in monodisperse bead packs. First, direct visual observations identify the following generation mechanisms: lamella leave-behind, gas-bubble snap-off, and lamella division. Then, to ascertain the relative importance of these mechanisms, quantitative experiments are pursued on the role of bead- pack permeability (bead sizes from 0.25 to 1 mm [0.01 to 0.04 in.]), gas-phase velocity (0.001 to 0.8 cm/s [0.0004 to 0.3 in./sec]), gas-phase fractional flow (0.60 to 1.0), permeability variations, and surfactant type [sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), Chevron Chaser SD1000™, and Suntech IV 1035™]. We discover a critical velocity, above which a "strong" foam is generated and below which only "weak" foam is formed. The snap-off mechanism is the primary mechanism responsible for the formation of the strong foam. A simple model, based on the concept of a "germination site," is developed to predict the onset of snap-off at higher gas velocities. New experimental data obtained in the homogeneous glass-bead packs for the critical capillary number necessary to form a strong foam are in excellent agreement with the proposed germination- site model.

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