Droplet vorticity alignment in model polymer blends

We utilize stroboscopic optical microscopy to monitor the shear induced def ormation of polymeric droplets in an immiscible polymeric matrix and find t hat under conditions of high droplet elasticity, the droplets can align in the vorticity direction. We consider the case where the viscosity ratio of the two phases is near unity but the elasticity ratio of the droplet to the matrix is greater than 100. This is achieved by using a matrix phase of po lydimethylsiloxane and a droplet phase of a polyisobutylene based "Boger" f luid. In the limit of weak shear and small droplets, the droplet alignment is along the shear direction, whereas for strong shear and large droplets, the alignment is along the vorticity direction. There is a range of conditi ons for which alignment can be along either axis. For droplets aligned alon g the vorticity axis, the distribution of aspect ratios is broad. The kinet ic transformation from droplet flow alignment to vorticity alignment upon i ncrease of shear flow has been observed, as well as the relaxation back to a spherical shape upon cessation of shear. We deduce from the kinetic exper iments that the thickness in the gradient direction is less than that in th e flow and vorticity directions. (C) 2000 The Society of Rheology. [S0148-6 055(00)00202-9].