The Importance of Oscillatory Structural Forces in the Sedimentation of a Binary Hard-Sphere Colloidal Suspension

Concentrated colloidal dispersions are complex systems comprised of colloidal particles of various sizes. This paper summarizes recent experimental and theoretical findings oil understanding the interactions between microspheres in the presence of submicrometer particles; the submicrometer particles can be solvent molecules, surfactant micelles, or other nanometer-sized particles, In a hard-sphere suspension containing microspheres in the presence of submicrometer particles, oscillatory structural (entropic) forces between large particles, caused by small particles, arise as a result of the collective particle-particle interactions. The magnitude of the oscillatory structural forces is theoretically estimated based on the statistical mechanics approach, Monte Carlo simulations, and stochastic particle dynamics methods. Also, sedimentation experiments using spherical low-charged, binary hard-sphere particle suspensions with a large size ratio are presented to elucidate the importance of the oscillatory structural forces (i.e., attractive depletion and repulsive structural forces) in particle dispersion stability, Micronmeter-sized, low-charge latex particle concentration profiles in the presence of nanoparticles (nonionic micelles) were monitored using a nondestructive Kossel diffraction technique. To rationalize the experimental observations, the particle settling dynamics were simulated by considering the hydrodynamic interactions, structural forces, and Brownian motion in a gravity field. The theoretical predictions were determined to be in satisfactory agreement with the experimental observations. The effect of the particle size ratio (i.e., micrometer to submicrometer) on interparticle interaction energy is highlighted.