Influence of confinement on the steady state behaviour of single droplets in shear flow for blends with one viscoelastic component

By using a counter rotating plate-plate device, single droplets in shear flow have been microscopically studied at confinement ratios ranging from 0.1 to 0.75. The droplet-to-matrix viscosity ratio was fixed at 0.45 and 1.5. Results are presented for systems with a viscoelastic Boger fluid matrix or a viscoelastic Boger fluid droplet, at a Deborah number of 1. Although the separate effects of confinement and component viscoelasticity on dropletdynamics in shear flow are widely studied, we present the first systematic experimental results on confined dropletdeformation and orientation in systems with viscoelastic components. Above a confinement ratio of 0.3, wall effects cause an increase in dropletdeformation and orientation, similar to fully Newtonian systems. To describe the experimental data, the Shapira–Haber theory [Shapira, M., and S. Haber, Int. J. Multiph. Flow16, 305–321 (1990)] for confined slightly deformeddroplets in Newtonian-Newtonian systems is combined with phenomenological bulk models for systems containing viscoelastic components [Maffettone, P. L., and F. Greco, J. Rheol48, 83–100 (2004); M. Minale, J. Non-Newtonian Fluid Mech.123, 151–160 (2004)]. The experimental results are also compared to a recent model for confined dropletdynamics in fully Newtonian systems [M. Minale, Rheol. Acta47, 667–675 (2008)]. For different values of the viscosity ratio, component viscoelasticity and Ca-number, good agreement was generally obtained between experimental results and predictions of one or more models. However, none of the models can accurately describe all experimental data for the whole range of parameter values.