Oscillatory shear induced droplet deformation and breakup in immiscible polymer blends

Abstract

Deformation and breakup of droplets in polybutadiene/polydimethylsiloxane blends subject to oscillatory shear flow were investigated experimentally using an optical shear flow cell. The apparent major axis (a∗) and the minor axis (c) in the vorticity direction of the droplets were measured as functions of time. From the time series of a∗ and c and the deformation parameter, (a∗−c)/(a∗+c), we define the deformation amplitudes as one-half the differences between the maximum and minimum values. The deformation amplitude parameters generally decrease with increasing viscosity ratio, time scale ratio, and droplet elasticity. The dependences of the deformation amplitude parameters on capillary number are generally linear up to a certain value for Newtonian droplets regardless of viscosity ratio and time scale ratio. The dependences become totally nonlinear with increasing droplet elasticity. Droplet viscosity and elasticity generally impede breakup under oscillatory shear. Critical capillary number for breakup, the number of resultant daughter droplets, and the number of cycle required for breakup to occur increase with time scale ratio. The apparent breakup pattern changes from the dumbbell type to the end-pinching type as time scale ratio increases.