Hadi Mohammadigoushki, Giovanni Ghigliotti & James J. Feng
Phys. Rev. E 85, 066301 (2012)
Abstract - We report an experimental study on shearing a monolayer of
monodisperse bubbles floating on liquid in a narrow-gap Couette
device. The bubbles in such a "bubble raft" coalesce only if the
shear rate exceeds a threshold value. This is in contrast to the
conventional wisdom that bubbles and drops coalesce for gentler
collisions, at shear rates below a critical value. Furthermore,
the threshold shear rate increases with the bubble size and the
viscosity of the suspending liquid, contravening reasoning based
on capillary number. Through visualization and scaling arguments,
we investigate several plausible mechanisms for the anomalous coalescence.
None explains all aspects of the observations.
The most promising model is one based on inertial forces that compress
the bubbles radially inward and accelerate film drainage.