Pearling instabilty: Bubble moving up in a viscous fluid
Air bubble moving up in viscous fluid leaves “long-thin-air strands” (Picture 2) in the fluid. After some time this long air strand breaks of into very very small bubble separated by small distances( <0>
The formation of long-thin-strands from main bubble rising up is a bit different case as compared to regular atomization. My experiment is similar to the case of water jet breakup from a dripping faucet, but with fluids exchanged. Here the buoyancy force is additional to other competing forces like surface-tension, gravity, viscosity etc. In ‘regular’ atomization, necking, threading and pinching off of droplet happens. In this case, necking and pinching off does not occur. Rather air from the main bubble leads to formation of elongated air strands. depending on density and viscosity, many a times air-strands do not form but a sharp peak can be observed. After discussion with my professor, I learned that It appears that the strand of air is due to `pearling’, an instability similar to that in drops when they move on surfaces. The high viscosity in this case may be making the detachment from the backside of the bubble into a strand instead of the usually occurring bubbles/drops.
There is a lot not understood about pearling even in simple fluids, I am sure there must be more unknown in non-newtonian high viscosity fluids.
(Picture 3): The separation of the strand into bubbles could be due to the usual capillary instability, again modified by the high viscosity.
Gaurav Singh, Graduate student at Indian Institute of Technology, Madras.