Boundary effects on forced drainage through aqueous foam

The flow of liquid through foam confined in vertical tubes was investigated by measuring the velocity v(f) of the liquid front forced down by gravity for various flow rates Q. The power law relating the velocity to how rate of the incoming liquid (v(f) proportional to Q(alpha)) was observed for tubes of various cross-sectional areas A. The exponent alpha was found to vary linearly with the reciprocal of the area: alpha = 0.325 + 13.7 mm(2)/A. This further supports the node-dominated foam drainage model, which predicts alpha = 1/3 in the limit of infinite cross-sectional area. This relation appears to be independent of bubble size, suggesting that using smaller foam bubbles may not alleviate boundary effects. The results of these experiments also partially explain the discrepancies in measurements of alpha reported in previous studies.