Simultaneous Breakup of Multiple Threads in a Micro-Confined Emulsion

Jai A. Pathak, Polymers Division, NIST, Gaithersburg, MD 20899-8544

We have studied the simultaneous breakup of multiple strings formed under appropriate conditions (known from previous work) in-situ under confinement in model emulsions of polyisobutylene, PIB and poly(dimethylsiloxane), PDMS. Our experiment enables us to discern the interplay between confinement and hydrodynamic interactions between strings. Upon cessation of flow, the time-to-break of a string is controlled by its degree of confinement (the ratio of gap width between the parallel plates and the unperturbed diameter of the thread). The time-to-break of a given string increases with its degree of confinement. Hydrodynamic interactions strongly influence the rate of growth of instability on neighboring strings.  Neighboring strings, when sufficiently close, break up by an out-of-phase mechanism, wherein the resultant droplets from neighboring strings are formed one-half wavelength away from each other. In contrast to recent reports of in-phase break up in extremely close neighboring strings, we do not see any in-phase breakup.  We see the formation of satellite and sub-satellite droplets in addition to daughter droplets upon breakup, which signifies the strong non-linearity of the late stages of the breakup process. We find that the wavelength of the Rayleigh-Taylor instability is higher under confinement.  Combination of this work and previous work from our group suggests that while confinement slows down the time rate of amplitude growth (relative to the bulk case) in the early stages, the growth rate in the late stages of the instability in the bulk and confined cases is the same.