Uniform foam production by turbulent mixing: new results on free drainage vs. liquid content

An apparatus is described for rapidly producing large quantities of foam via turbulent mixing of gas with a narrow jet of a surfactant solution inside a delivery tube. By controlling relative flow rates, the gas volume fraction in the resulting foam may be easily varied across 0.3 < phi < 0.99. Using such foams, we present a comprehensive set of data for free drainage as a systematic function of gas fraction and sample geometry. The qualitative behavior can be understood in terms of simple theoretical considerations, emphasizing the importance of controlling the initial foam conditions. Quantitative features are compared with two approximate versions of the drainage equation, highlighting the crucial role of capillarity for very dry foams and small samples.