Foams are a special type of complex fluids with unusual flow properties. Macroscopically, foams often exhibit a yield stress and aging behavior. The physical mechanisms are to be sought from the bubble morphology and packing on the microscopic level. We approach the problem from the viewpoint of interfacial fluid dynamics. We make a monolayer of bubbles, often called a bubble raft, floating on liquid water, and then subject the 2D foam to steady shearing. The goal of the project is to elucidate the coupling between microstructural evolution and rheology of the foam as a whole. In particular, we are exploring the interaction among neighboring bubbles as well as long-range "collective deformation" among bubbles. The images and video below illustrate typical morphology of sheared bubble rafts.

(The left panel links to a video segment showing bubble motion during shearing. The right panel shows the velocity vector field measured using Particle Image Velocimetry or PIV.)

In a narrow-gap Couette shear cell, we have discovered a new mechanism for bubble coalescence under vigorous shearing (full text). If a larger bubble is placed in an otherwise monodisperse 2D foam of smaller bubbles, it migrates to the center of the gap (full text) much as a single droplet would when suspended in a viscous liquid. Moreoever, we found that in sheared polydisperse foams the bubbles tend to segregate according to size.