Unusual Characteristics of the Maximum Bubble Pressure Method Using a Teflon Capillary

Abstract

The maximum bubble pressure method has been used recently in a continuous-bubbling mode to measure the dynamic surface tension of surfactant solutions at varying bubble frequencies. A modification of this method, which utilizes a syringe pump supplying gas at controlled rates, has been developed to examine bubble growth by providing an initially stationary gas-liquid interface. Videomicroscopy results indicate that the bubble-growth rate is not constant, and the bubble-growth time from a stationary interface at the tip of a Teflon capillary to a hemispherical shape may be only a small fraction of the "peak-to-peak" time for some concentrations of surfactant solution. In these cases, bubble generation occurs in three steps: growth at the capillary tip, rapid expansion at the end of the tip, followed by repressurization of the system. Of particular significance is the observation that the first of a series of bubbles, generated in surfactant solutions at and below the critical micelle concentration (CMC), has a lower dynamic surface tension than subsequent bubbles. This result is attributed to the initial equilibrium condition of the interface.