Influence of confinement on the steady state behavior of single droplets in shear flow for immiscible 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 droplet dynamics in shear flow are widely studied, we present the first systematic experimental results on confined droplet deformation and orientation in systems with viscoelastic components. Above a confinement ratio of 0.3, wall effects cause an increase in droplet deformation and orientation, similar to fully Newtonian systems. To describe the experimental data, the Shapira-Haber theory [Shapira, M., and S. Haber, Int. J. Multiph. Flow 16, 305-321 (1990)] for confined slightly deformed droplets in Newtonian-Newtonian systems is combined with phenomenological bulk models for systems containing viscoelastic components [Maffettone, P. L., and F. Greco, J. Rheol 48, 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 droplet dynamics in fully Newtonian systems [M. Minale, Rheol. Acta 47, 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. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3236837]