Stepwise thickening in aqueous foam films stabilized by sodium naphthenates

During drainage of a foam film formed from an aqueous sodium naphthenate solution, a transient, local, stepwise thickening process was observed. Film stratification is related to the stepwise thinning drainage process where individual layers of material are drained from a film. The process typically involves the appearance of a sequence of small, uniformly thick spots that eventually expand to the size of the film. The appearance and growth of each spot represents a discrete decrease in the thickness of the film. The size of each decrease or step typically corresponds to the size of one or more lamella layers of the stratified film. Stepwise thinning was observed in a foam film formed from an aqueous sodium naphthenate solution, as frequently reported for a variety of systems. However, during the drainage process, a transient stepwise thickening process was also observed. Bright spots began to appear and grow, indicate a discrete increase in the thickness of a portion of the film. This local, stepwise thickening process appeared to be an alternate and temporary drainage process directly related to the expansion of the stepwise thinning spots.