Light-scattering experiments on phase-separation dynamics in binary fluid mixtures

Abstract

We present the method and results of two phase-separation experiments performed on the binary-polymer blend polyisoprene-poly(ethylene-propylene). Elastic light scattering and optical microscopy were used to follow the phase separation after quenches into the coexistence region of the phase diagram. For off-critical quenches, we observed the formation of spherical droplets which obtained nearly monodisperse radius distributions over some fraction of the free-growth period. These results are interpreted within the framework of a heterogeneous nucleation process and diffusion-driven dynamics. For critical quenches, we observed the well-known bicontinuous infinite-cluster morphology normally associated with spinodal decomposition, but with two distinct growth modes. In the bulk, we observed L(t)∼${\mathit{t}}^{1/3}$, which is consistent with diffusion-driven dynamics, whereas near the sample walls we obtained a fast growth with L(t)∼${\mathit{t}}^{3/2}$, inconsistent with either diffusion- or interface-driven dynamics. We attribute this large exponent to wetting effects, but still lack any theoretical understanding of the phonomenon.