The behaviors of time-dependent interaction between two buoyancy-driven in-line deformable drops rising in pure glycerin at

Re

1 were studied

where the diameter for the leading drops ranged from 9.51 mm to 12.6 mm and for trailing drops from 12.7 mm to 15.8 mm. The situation while a larger drop chasing a smaller onewas specifically consideredwhich typically led to the smaller drop "coating" the larger one. Two approaches

the geometric feature approach and the energy change one

were used to judge the starting and ending times of the interaction between twodrops. Based on a conicalwakemodel

the drag coefficient of twoapproaching drops was calculated. Due to the approaching effect of the trailing drop

the leading drop was accelerated and the average drag coefficientwas smaller than that for a free rising single drop. The frequency spectrums of the lateral oscillation of two drops during the interaction were obtained by using Fourier analysis. The oscillation frequency of the interactional drops was also different from that for a free rising single drop because of the wake effect produced by the leading drop. Due to a superposition of the drop shape oscillation and the drop internal circulation

the transverse

oscillation frequencies of two drops have the same trend during the approaching process.