Interparticle interactions and direct imaging of colloidal phases assembled from microsphere-nanoparticle mixtures

We investigate the interparticle interactions, phase behavior, and structure of microsphere-nanoparticle mixtures that possess high size and charge asymmetry. ¹ We employ a novel Monte Carlo simulation scheme ² to calculate the effective microsphere interactions in suspension, yielding new insight into the origin of the experimentally observed behavior. ³ The initial settling velocity, final sediment density, and three-dimensional structure of colloidal phases assembled from these binary mixtures via gravitational settling of silica microspheres in water and index-matched solutions exhibit a strong compositional dependence. Confocal laser scanning microscopy is used to directly image and quantify their structural evolution during assembly. Below a lower critical nanoparticle volume fraction (φ _nano < φ ^L,C), the intrinsic van der Waals attraction between microspheres leads to the formation of colloidal gels. These gels exhibit enhanced consolidation as φ _nano approaches φ ^L,C. When φ _nano exceeds φ ^L,C, an effective repulsion arises between microspheres due to the formation of a dynamic nanoparticle halo around the colloids. From this stable fluid phase, the microspheres settle into a crystalline array. Finally, above an upper critical nanoparticle volume fraction (φ _nano > φ ^U,C), colloidal gels form whose structure becomes more open with increasing nanoparticle concentration due to the emergence of an effective microsphere attraction, ³ whose magnitude exhibits a superlinear dependence on φ _nano.