Droplet-String Deformation and Stability During Micro-Confined Shear Flow
Jai A. Pathak, Kalman D. Migler
We have performed experiments on model emulsions of polyisobutylene (PIB) and poly(dimethylsiloxane) (PDMS) to quantify the effect of confinement on deformation and stability of droplets and strings (threads) under flow. It is known from some of our earlier work that under confinment, droplets in concentrated emulsions can coalesce with each other to form strings, and in the present context strings can be simply viewed as droplets having a large aspect ratio. Whereas in the bulk case, there are two known states of a droplet (stable or unstable) as determined by the droplet Capillary number (Ca), we find that confinement induces three additional states: squashed drops, stable strings and unstable strings. For strings, deformation under confinement is a very string function of Ca: the aspect ratio of a string scales as the cube of Ca. This scaling relationship is unique to confinement, and it sets strings apart from transiently stretched droplets in the bulk. Confinement no only promotes deformation, but also allows larger stable droplets to exist under flow that what is predicted by the critical Ca. Strings are stabilized by a combination of the shear flow field and wall effects aring from confinement.