Effect of polymer-coated gold nanoparticle stabilizers on drop coalescence

Abstract

Polymer-coated gold nanoparticles (PGNPs) can be used as stabilizers in immiscible polymer blends, similar to block-copolymers (BCs). However, the PGNP gold cores increase the magnitude of the disjoining pressure (Π), i.e., the van der Waals interaction for unit area, in the film between the drops, favoring coalescence. This might explain the counterintuitive 70% drainage time (td) reduction for polymeric drops stabilized by PGNPs compared to those stabilized by BCs, as reported in recent flow-induced head-on collision experiments in extensional flow, despite PGNPs being more surface active. Knowledge of the mechanisms determining td is fundamental for designing effective PGNP compatibilizers. Here, we performed a parametric study of those experiments via boundary integral simulations, treating PGNPs as surfactants and utilizing for the first time a disjoining pressure expression which includes the effect of interfacial PGNPs (ΠPGNP). In particular, we varied the PGNP concentration and core size in ΠPGNP, the surface diffusivity (Ds) via the surface Peclet number, and the surface elasticity via the Marangoni number. Flow-induced coalescence was very sensitive to all three parameters. td was reduced up to 60% for touching 3 nm core diameter PGNPs, increasing significantly the coalescence probability for drop sizes <5 µm, but the soft coronas diminished this effect considerably. Thus, other causes, besides the enhanced Π, had to be simultaneously present to explain the dramatic experimental td reduction; the most likely is a Ds higher than its Stokes-Einstein relation estimate and the PGNP ligands being in a dry-brush regime, leading to entropic attraction between the drop interfaces.