| Oxygen bubbles produced electolytically on an electrode are reported to be attractive. According to the thermocapillary flow model developed by Guelcher et al. (1998), aggregation of bubbles near a wall under temperature gradient normal to the wall is caused by thermocapillary flow about the bubbles. It is therefore predicted that the aggregating velocity of the bubbles scales linearly with the temperature gradient, the size of the bubbles and the inverse of the viscosity of the liquid. An experimental cell was designed and experimental analysis of this phenomenon was performed. Trajectories of pairs of bubbles aggregating were shown to scale linearly with the temperature gradient, the size of the bubble and the viscosity of the liquid, as predicted by the thermocapillary flow model. The motion of the bubbles parallel to the wall is described by a hindrance parameter that was determined both experimentally and theoretically. With this hindrance parameter the thermocapillary flow model can predict the trajectories of the bubbles.; However, because this model is based on thermocapillary flow around a single bubble, the model fails to explain an experimentally observed increase of aggregation velocity when the bubbles close to each other. Another model including the presence of two bubbles has been investigated numerically to explain the increase of the velocity. A commercial software package Fluent with the Finite Volume Method for solving momentum and heat transfer equations was implemented for the simulation. Both attractive force on the bubble and hindrance to the motion of the bubble increase with the approach, but the increase in attractive force is more significant than the increased hindrance, leading to the increase in the velocity. Although close quantitative agreement between the experimental and calculated trajectories for the center-to-center distance between the bubbles has not been demonstrated because of the possible deformation of the bubbles, the increase in the aggregation velocity with small center-to-center distance is well described by this two-bubble thermocapillary flow model and the minimum and maximum velocities calculated bracket the data. |
