Natural surfactant system: precursor flow in one-shot two-dimensional foam drainage.

Foam drainage, the downward flow of the intervening liquid due to gravity, surface tension and viscosity, is an important factor for foam stability. Forced drainage (adding liquid to a foam column from the top) is used to study liquid flow characteristics in foam. The different competing phenomena involved in the vertical and horizontal movement is studied by visually monitoring the draining liquid using a dye.