Induced Attraction between Polystyrene Colloidal Particles in a Binary Mixture with PNIPAM Colloidal Microgels

We investigate the phase behaviors of binary mixtures of polystyrene (PS) hard-sphere and poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM) soft-sphere colloidal particles as a function of temperature. As the temperature increases, apparent attractions arise between the PS particles, to the point of clustering at the highest temperature. This attractive force is found to be due to the change in pH caused by the acrylic acid co-polymerized with the temperature-sensitive PNIPAM particles, which in turn collapses the sterical stabilizing surface layers on the PS particles. Our experiments provide a new way to tune colloidal interactions with temperature for particles that are otherwise insensitive to temperature variations.