title: Anomalous coalescence in sheared two-dimensional foam.
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.
authors.affiliation: Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, V6T 1Z3 Canada.
authors.forename: Hadi
authors.initials: H
authors.lastname: Mohammadigoushki
authors.forename: Giovanni
authors.initials: G
authors.lastname: Ghigliotti
authors.forename: James J
authors.initials: JJ
authors.lastname: Feng
mesh_terms: D003198:Computer Simulation; D005740:Gases / Q000737:chemistry*; D008956:Models, Chemical; D012212:Rheology / Q000379:methods*; D033081:Shear Strength; D012996:Solutions / Q000737:chemistry*
chemical_list: D005740:Gases; D012996:Solutions