Dynamics of annular solvent droplets under capillary thinning of non-entangled polymer solution

Formation and coalescence of solvent droplets on a thread of polymer solution at the final stage of capillary pinching is studied theoretically. It is considered that macromolecules are already almost completely stretched along the extension axis and their contour length exceeds the diameter of the thread. In this regime, the radius of polymer string decreases slowly with time under the action of capillary forces and the solvent squeezes out to the thread surface forming annular droplets of different sizes. The thinning process stops when the capillary pressure is balanced by the osmotic pressure of the polymer. As a result, a quasistationary two-phase structure of polydisperse solvent droplets on a polymer string is formed. We develop a rigorous theory showing that the polymer core is swollen in the droplet regions but still remains much thinner than the solvent phase. We also demonstrate that such a blistering structure is unstable with respect to droplet coalescence and elucidate two mechanisms of this process due to the solvent flow between the droplets and due to diffusion of solvent droplets along the polymer string. Both mechanisms lead to the same long-time power law ( t1/7) for the droplet radius. It is shown that a breakage of the polymer string may occur at time scales exceeding the Rouse time of polymer chains.