Drop deformation and breakup in confined flows of viscoelastic two-phase systems

Deformation and breakup of fluid particles translating through capillaries of different cross-section are investigated experimentally for systems in which either the drop or the suspending fluid is rheologically complex. Capillaries with circular, square/rectangular, and periodically-varying cross-sections are considered, and the effect of the confining geometry on breakup behavior is examined. Experimental observations of the steady drop shape and mobility are reported for various polymer concentrations in the rheologically complex phase in order to identify the effect of elasticity on fluid particle deformation and breakup.;Viscoelastic effects on fluid particle deformation are examined over a wide range of elasticity numbers. Elasticity of the suspending fluid is found to produce steady cusped tails at the trailing end of bubbles, similar to that reported by other investigators for the free rise of large bubbles in a viscoelastic suspending fluid. Elasticity of the drop fluid is found to hinder drop deformation while elasticity of the suspending fluid seems to have a non-monotonic effect on drop deformation. Qualitatively similar effects of elasticity were observed for both cylindrical and rectangular channels. For the periodically-constricted capillary, elasticity in either phase (drop or suspending fluid) is found to enhance deformation.;The effects of capillary number, drop size and viscosity ratio on the relative drop mobility in different capillaries are also examined. The steady relative velocity of the drop is found to be an increasing function of capillary number that approaches a constant value for sufficiently large capillary numbers. Drop mobility is retarded by increasing drop phase elasticity in all of the capillary geometries considered. Suspending fluid elasticity on the other hand, has a favorable effect on drop mobility in a periodically-constricted capillary, and a non-monotonic effect on drop mobility in both cylindrical and rectangular channels.;Two distinct modes of drop breakup were observed for pressure-driven motion of viscoelastic drops through a Newtonian suspending fluid within a cylindrical capillary; growing re-entrant cavity and rim-streaming. We observed a double tip-streaming mode of drop breakup in the rectangular channel, and re-entrant cusp streaming in the square channel. For both the cylindrical tube and the rectangular channel, increasing the elasticity of the drop phase (by increasing the polymer concentration) initially facilitated drop breakup, and shifted the onset of drop breakup to smaller drop sizes. As the elasticity number was increased beyond about 0.5, however, the onset of drop breakup was delayed to larger drop sizes due to the concomitant increase in the viscosity of the drop phase. In the periodically-constricted capillary, the tail pinchoff and re-entrant cusp modes of drop breakup were observed for weakly elastic and moderately/strongly elastic systems, respectively. For pressure-driven motion of Newtonian drops through a viscoelastic suspending fluid, the tail-streaming and tail pinch-off breakup mechanisms were observed in the cylindrical and periodically-constricted capillaries, respectively, while no breakup was observed in the rectangular channel over the range of parameter values considered in the experiments.