CURRENT AND RECENT PROJECTS
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.
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 (full text). This segregation produces temporal evolution in the overall rheology of the foam, in an interesting example of structure-rheology correlation. Furthermore, shearing 2D foam in a wide-gap device brings forth non-Newtonian (shear-thinning and normal-stress) effects (full text), which affect bubble migration in return.