Novel Microstructure in Polymer Blends Sheared Under Confinement
Jai A. Pathak, Kalman D. Migler
In this review article, we discuss the implications for the morphology of an immiscible polymer blend, or in general, an emulsion, in the limit when the two length scales, viz., the average droplet size and characteristic experimental length scale, become comparable to each other. This situation is easily realized when the average droplet size in the emulsion becomes on the order of the gap width between parallel platens in a flow visualization shear cell consisting of optically clear parallel platens. We had earlier discovered that a morphological droplet-string transition, driven by the interplay between coalescence, breakup and finite-size effects, occurs in polymer blends under conditions of droplet confinement. The droplets coalesce with each other in the flow direction to form strings.The deformation (aspect ratio) of these strings scales very strongly, approximately as the cube of the Capillary number, and they form under conditions that correspond to super-critical Capillary numbers for a single droplet. Thus, these strings lie in a new regime of deformation that is certainly unknown in droplets in the bulk. Confinement enhances deformation and promotes stability. In addition, these strings persist for very long times upon cessation of shear, certainly much longer than they would persist in the bulk. The interplay of confinement and flow causes a suppression of the Rayleigh-Taylor-Tomotika instability. Confinement causes other fascinating micro-structural transitions in flowing blends, such as layering of droplets in the flow-velocity gradient plane.