The deformation of moving drop plays an important role in its evaporation and combustion. Surface tension is one of the factors which influence the deformation. For investigating the effect of surface tension on the deformation

the low concentration SDBS surfactant water based solutions with surface tension in the range of 30 to 72 mN·m-1 are used as the experimental liquid. The different sizes of needles are employed to get drops with diameter at 3-5 mm. The image processing is utilized to analyze the drop shape by using a high speed camera (Phantom V211

1000 pps

800×600 pixel). The complete drop falling image process is assembled by 9 sections at different position with different drops. A stable drop formation time of (2.4±0.1) s makes the dispersion of size and initial velocity of these drops meet the experimental accuracy. Analyzing the experimental data by Image-pro plus obtains the semi-empirical correlations of Eötvös number (Eo). The results show that the period and amplitude of drop deformation decrease with increasing surface tension. Furthermore

the momentum transfer inside the drop causes initial transient impulse

which makes the drop periodic deformation when it falls down.