Quantitative measurements of liquid holdup and drainage in foam using NMRI

Abstract

Liquid fraction profiles of a column of draining aqueous foam stabilized with sodium dodecyl sulfate were studied by nuclear magnetic resonance imaging (NMRI) at high spatial and temporal resolution. It was observed that the liquid holdup in the column is not locally homogeneous in the direction of the column axis with liquid holdup exhibiting periodic variation in time. This creates ‘‘ripples’’ in the liquid content profiles (that is, an approximately sinusoidal variation of liquid fraction in space). These ripples are seen to actually rise in the column as liquid drains down through the foam. This behavior may be explained by a simple mass balance: the rising velocity of the ripples multiplied by the gas fraction in the foam is equivalent to the liquid superficial drainage rate. Thus, NMRI is proposed as a powerful tool for studying drainage of foams because it provides a noninvasive method of measuring superficial drainage rate in foams that yields time-resolved instantaneous drainage rates as a function of position in the column. The measured liquid drainage rates were compared with a dimensionless expression for the liquid drainage rate and excellent agreement was achieved.