We report on the numerical implementation of thin-film equations that describe the capillary-driven evolution of viscous films, in two-dimensional configurations. After recalling the general forms and features of these equations, we focus on two particular cases inspired by experiments: the leveling of a step at the free surface of a polymer film, and the leveling of a polymer droplet over an identical film. In each case, we first discuss the long-term self-similar regime reached by the numerical solution before comparing it to the experimental profile. The agreement between theory and experiment is excellent, thus providing a versatile probe for nanorheology of viscous liquids in thin-film geometries.
We present results on the leveling of polymer microdroplets on thin films prepared from the same material. In particular, we explore the crossover from a droplet spreading on an infinitesimally thin film (Tanner's law regime) to that of a droplet leveling on a film thicker than the droplet itself. In both regimes, the droplet's excess surface area decreases towards the equilibrium configuration of a flat liquid film, but with a different power law in time. Additionally, the characteristic leveling time depends on molecular properties, the size of the droplet, and the thickness of the underlying film. Flow within the film makes this system fundamentally different from a droplet spreading on a solid surface. We thus develop a theoretical model based on thin film hydrodynamics that quantitatively describes the observed crossover between the two leveling regimes.
Since short polymer chains have a higher mobility than long molecules, conventional expectations are that the growth rate, G, of polymer crystals should decrease as the concentration of large chains increases in a binary blend.Here we present results on G as the blend concentration, ϕ, is varied from short chains of poly(ethylene oxide) (PEO), which are well above the entanglement molecular weight, to long PEO chains. Contrary to the simple mobility argument, G(ϕ) is nonmonotonicclear evidence that another mechanism can dominate. We propose a tentative model based on the simple idea that chain ends retard the crystal growth. Thus, increasing the chain end concentration with the addition of short chain molecules can reduce the crystal growth rate.
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