Drop shape dynamics under shear-flow reversal

The shape evolution of a liquid drop immersed in an immiscible liquid is st udied under transient flow conditions. The drop is Newtonian and buoyancy f ree; the external liquid is Newtonian and subjected to shear flow reversal. Three model systems, polydimethylsiloxane/polyisobutylene, polybutene/sili cone oil, and silicone oil/polybutene, all Newtonian under the experimental conditions investigated, have been selected to have a range of viscosity r atios. The three drop axes and the drop orientation within the shear plane are independently measured. The results are compared with the predictions o f a phenomenological model. The agreement between experimental results and theory is good. Peculiar behavior of the orientation angle has been observe d and correctly predicted. The results are also used to explain some rheolo gical features typical of immiscible polymer blends. (C) 2000 The Society o f Rheology. [S0148-6055(00)00806-3].