(246c) Effect On Nonadsorbed Nanoparticles On near-Contact Dynamic Interactions Between Surfaces

Colloidal probe atomic force microscopy was used to measure the effect of scan rate on the interaction between a micron-sized colloidal particle (test particle) and a flat plate in aqueous solutions containing dispersed spherical nanoparticles. The nanoparticle concentrations investigated were sufficiently large that long-range oscillations in the force profile were clearly observed, suggesting ordering of the nanoparticles in the confined region between the test particle and plate. The measurements allowed investigating (1) the effect of scan rate (rate of approach or retraction between the test particle and plate) on the nanoparticle ordering, and (2) the effect of the nanoparticles on the hydrodynamic drag force exerted on the test particle. To understand the effect of scan rate on nanoparticle ordering, a dimensionless Peclet number for the gap region was developed and evaluated. For the effect of the nanoparticles on the test particle drag force, it was found that even at nanoparticle volume fractions of several percent by volume, the drag force exerted on the test particle was well described using equations developed for simple Newtonian fluids.