String formation in sheared polymer blends: coalescence, breakup, and finite size effects.

We have discovered a droplet-string transition in concentrated polymer blends which occurs when the size of the dispersed droplets becomes comparable to the gap width between the shearing surfaces. The transition is abrupt and proceeds via the coalescence of droplets in a four-stage kinetic process. Once formed, the strings are stable and exhibit pronounced hysteresis. The string state is stabilized by a suppression of the Rayleigh-Tomotika instability due to both finite size effects and to the shear-induced advection of small-amplitude disturbances.