Interactions between two bubbles under isothermal conditions and in a downward temperature gradient

Results from experiments on the motion of a pair of air bubbles in a silicone oil under isothermal conditions as well as in a downward temperature gradient are reported. Three situations were investigated: motion driven by buoyancy, motion dominated by thermocapillarity, and motion when the gravitational force and the thermocapillary force on the larger bubble of the pair were comparable. The experimental results were found to be in good agreement with theoretical predictions from the method of reflections. When the gravitational force was slightly smaller than the thermocapillary force on the larger bubble in the pair, the larger bubble moved downward but the smaller bubble nearby was found to move upward. This counterintuitive behavior is actually consistent with expectation, and an explanation is given.