On the coalescence of miscible drops and the pinch-off of a bubble

S. T. Thoroddsen
Mechanical Engineering, National University of Singapore

We will present high-speed imaging of three free-surface flows.

First, we look at the coalescence of two liquid masses which have different liquid properties. The focus is on a difference in the surface tension, for example during the coalescence of a water drop with a layer of ethanol. The resulting Marangoni stress produces waves which travel along the drop with the higher surface tension. The self-similar character of these waves will be studied for drops as small as 1 mm. On the other hand, we find that the weaker of the two surface tensions controls the coalescence speed, which is measured by the growth in radius of the connecting neck region.

Secondly, the pinch-off of a bubble will be imaged at frame-rates as high as 1 million fps. An air bubble is grown slowly at a vertical nozzle until it becomes unstable and pinches off. The necking region is studied at resolutions of about 10 micron/pixel. The very high frame-rate allows us to pinpoint the pinch-off time to about 1 microsec. We study the power-law for the decrease in neck radius, as the radius approaches zero. For gas in water the exponent is found to be slightly steeper than the 1/2predicted by inviscid dimensional analysis. For more viscous liquids a thin thread of air develops and breaks up into a string of bubbles. This work is similar to that of Burton et al. Phys. Rev. Lett., 94, paper no. 184502 (2005) and is consistent with most of their results, but shows smaller neck radii than they propose.

Finally, most in line with the topic of this workshop, some videos will be shown of the 'puncturing' of a drop. This occurs when a soap layer on the drop is compressed and 'buckles', sending out a jet of fluid. This configuration is produced when a pure water drop impacts onto and passes through a hemispherical soap film.