(186u) Drop Breakup in Viscoelastic Flow through Periodically Constricted Capillaries

We present the results of an experimental study of the deformation and breakup of bubbles and viscous/viscoelastic drops translating through a Newtonian/viscoelastic fluid within periodically-constricted capillaries. The capillary geometry is found to have a significant effect on drop mobility and deformation, as well as on breakup modes, in comparison with experimental results in a straight cylindrical capillary. The periodic variations of the cross-sectional area of the capillary cause the shape of the drop to be position-dependent. For low-concentration viscoleastic drops translating through a Newtonian fluid, as the drop reaches a constriction; its leading end follows the capillary wall and squeezes through the throat. As the capillary number increases in these systems, tail pinch-off is observed as the dominant break-up mode. As the drop moves through the constriction, the trailing end remains within the throat long enough to cause thinning of the midsection of the drop, followed by eventual pinch-off and formation of a satellite drop. The same breakup mode is observed for bubbles and viscous drops translating through a viscoelastic fluid. Reducing the elasticity of the suspending fluid delays the onset of breakup to higher capillary numbers. For high-concentration viscoelastic drops, large drops exhibit periodic formation of a re-entrant cavity. Increasing the drop size eventually leads to drop breakup through different breakup mechanisms. In these systems, a re-entrant cusp is observed at the trailing end of the drop. As the drop goes through the constriction, tipstreaming from the cusp into the drop eventually develops. Increasing the elasticity of the drop phase delays the onset of streaming.