Drop deformation in sheared polymer blends

Drop deformation in equiviscous polymer blends with dispersed phase volume fraction ranging from 0 to 10% has been investigated by video microscopy and image analysis in a parallel plate shear apparatus. Under steady external flow conditions the shape of individual drops, as measured by the three main axes, the orientation angle and the deformation parameter, showed marked time fluctuations around an average value depending on dispersed phase concentration and on drop capillary number. At small volume fractions the deformed shape was essentially coincident with the isolated drop case, whereas the higher the concentration, the larger the observed drop deformation at a given capillary number. Such deviations from the isolated drop case can be attributed to hydrodynamic interactions, mainly due to flow-induced collisions among drops. The experimental results can be conveniently described by a mean field approximation whereby the forces causing drop deformation are taken as proportional to blend viscosity rather than to continuous phase viscosity. By using this correction, i.e., by calculating the capillary number from blend viscosity, all the data collapsed to the isolated drop case, thus allowing one to exploit the small-deformation theoretical analyses available from the literature to predict the morphology of concentrated polymer blends.