(282b) Measurement of the Hydrodynamic Drag Force On a Microparticle Approaching a Flat Plate In a Nanoparticle Dispersion – Observation of a Separation-Dependent Effective Viscosity

The technique of colloidal probe atomic force microscopy was used to measure the hydrodynamic drag force exerted on a relatively large (30 μm diameter) spherical particle normally approaching a flat plate in aqueous solutions containing highly-charged silica nanoparticles (22 nm diameter). The objective was to determine the effect of the nanoparticles on the hindered mobility of the particle. In agreement with theoretical predictions of Bhattacharya and Blawzdziewicz (J. Chem. Phys 128, 214704, 2008) the effective viscosity experienced by the particle decreases at smaller separations as the nanoparticle concentration in the gap reduces. The effective viscosity equals that of the pure fluid at contact and becomes equal to the bulk viscosity only at surprisingly large separation distances (over 50 nanoparticle diameters!).  The effects of varying scan speed and ionic strength are also presented.