The Velocity of Ascending Air Bubbles in Aqueous Solutions of a Surface Active Substance and the Life of the Bubble on the Same Solution

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Abstract
1) The velocity of an air bubble ascending in water is remarkably retarded by the addition of a minute amount of sodium dodecyl sulfate (SDS) or isoamyl alcohol. With an air bubble with a radius ranging from 0.03 to 0.07 cm., the decrease in velocity is perceptible even in a 10−6 molar SDS solution, while it tends to a limiting value in 10−3 and 10−4 molar SDS solutions, where the rising velocity is about half as large as that in pure water. 2) To elucidate this retardation, the static properties of the solution, such as surface tension, density and compressibility, and the dynamic properties, such as viscosity and the change in surface tension with time, have been measured. However, no difference could be found in these properties between pure water and a 10−5 molar SDS solution to account for the retardation phenomena. 3) The rising velocity of a bubble considered to be a rigid sphere calculated by using an empirical formula fails to explain the results, since the formula does not fit our experimental conditions. The rising velocity of a rigid spherical bubble calculated from the observed data of a falling velocity of glass spheres in water in our apparatus is found to be in good agreement with that in 10−3 and 10−4 molar SDS solutions. 4) It has been concluded that, in all concentrations of the solution studied and for the bubble with a radius less than 0.07 cm., the bubble rises as a sphere and that the air circulation inside the bubble is closely related to the rising velocity of the bubble. The air circulation, which is strongest in the rising bubble in pure water, is hindered in 10−5 and 10−6 molar SDS solutions by the formation of an adsorbed film of SDS on this bubble, and the rising velocity is also thereby retarded. The circulation is actually suppressed, and the bubble rises as a rigid sphere in 10−3 and 10−4molar SDS solutions. The retardation of the rising velocity of bubbles with radii larger than 0.07 cm. is due to the deformation of the bubbles from a sphere. 5) The life time of bubbles on an aqueous, solution is affected by the surface active substance as sensitively as in the case of the effect of the same substance on the velocity of the rising bubble. Both phenomena are explained by the common factor of the retardatoin of the flow around the bubble surface.