Electrification of small air bubbles in water

Steady velocities of an air bubble moving along the axis of rotation of a glass cell filled with water were measured at two angles of tilt from a horizontal direction and at several rotational speeds. These velocities, many times smaller than Stokes' law would predict, can be explained by taking into consideration hydrodynamic forces which depend on density and the speed of rotation. An experimental determination of the law of resistance to the movement of a bubble in this rotating system leads to revised estimates of apparent surface electrical charge density inferred from measurements of electrical mobilities. These estimates, approximating 0.8 esu cm−2, are ten or more times larger than formerly reported. Furthermore, the surface charge density appears to be nearly independent of the bubble diameter. The electrification has been measured in a different manner with the same apparatus by establishing a static balance between the force of an electric field parallel to the axis of rotation and a component of the force of buoyancy. The charge carried by the bubble is not dependent upon its motion. The electrification of bubbles of air rising freely in water was measured also, and the magnitude of the apparent surface charge density has been found to be only about one-third that obtained with the rotating system under similar chemical conditions. This discrepancy is believed to be due to a lack of sufficient time for the rising air bubbles to attain an equilibrium electrified state. The observed effect of CO2 as a water impurity is to reduce the electrification of an air bubble, in agreement with the measurements of other investigators.