This paper is dedicated to Professor Gad Hetsroni on the occasion of his 65th birthday. Gad, through his leadership and devotion to science in general and multiphase¯ows in particular, has been the beacon of this ®eld. As the Founder and the Editor of the International Journal of Multiphase Flow, Gad has marched this ®eld forward at a rapid pace and positioned it at the focus of science and engineering. We would like to express our deep gratitude to Gad for all he has done for the ®eld of multiphase¯ow and we wish him many more productive and enjoyable years. AbstractThe motion of a single elongated (Taylor) bubble propagating in a transparent vertical pipe is studied experimentally in stagnant liquid, as well as in upward and downward liquid¯ow. Digital image processing of a sequence of video images serves as the main experimental method for the study of the Taylor bubble motion. In addition, the distribution of the velocities in front of the bubble and in the liquid ®lm is measured using Particle Image Velocimetry. The relation between the Taylor bubble motion and the velocity ®eld in front of it is discussed. #

The motion of elongated (Taylor) bubbles in air±water vertical¯ow is studied by processing consecutive series of digitized video images. The shape of the elongated bubbles, as well as the velocities of the liquid±gas interfaces, are measured for various¯ow conditions. The bubble velocity variation caused by air expansion under the in¯uence of hydrostatic pressure gradient is measured and compared with model predictions. An oscillatory motion of the bubble bottom is observed. The frequency and the amplitude of these oscillations are studied as a function of bubble length and water¯ow rate. This information is essential for accurate modeling of developing gas±liquid slug¯ow. #

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