Adhesion between Nanoscale Rough Surfaces.

Nanoscale surface roughness strongly affects the adhesion force between surfaces. In this investigation, a model that more accurately describes the size of an asperity based on the measurable parameters of root-mean-square (rms) roughness and the distance between the asperities is derived. The radius of the asperity from the proposed model is much larger than the radius used in previous approaches, considering the same surface with nanoscale roughness. Using the proposed geometry and previously suggested models, this paper elucidates the contributions from contact and noncontact interactions of a particle adhered to a surface with nanoscale roughness (approximately less than 20 nm rms). For most surfaces considered, the contact interaction of the asperity and the adhering particle are found to dominate the interaction. In the second paper of this series, the proposed model is compared to the experimentally determined force of adhesion in systems with nanoscale roughness. Copyright 2000 Academic Press.

Adhesion between Nanoscale Rough Surfaces.

In this investigation, the adhesion between particles and plates with root-mean-square, rms, surface roughness of 0.17-10.5 nm was measured by atomic force microscopy. Measurements obtained with particles both larger and smaller than the surface asperities are presented. Results indicate adhesion force decreases sharply with increasing surface roughness in the nanometer scale (<2 nm), followed by a gradual and slow decrease with further increase in roughness. Existing models were found to significantly underestimate adhesion force. Hence, a new model based on a geometry that considers both the height and breadth of asperities yielding an increased asperity radius compared to previous approaches, as detailed in Part I of this series, is applied using both van der Waals and elastic deformation/work of adhesion based approaches. For the system studied in this investigation, the adhesion forces predicted by the proposed model are considerably more accurate than those predicted by past models. Copyright 2000 Academic Press.

Role of Electrostatic Repulsion on the Viscosity of Bidisperse Silica Suspensions.

The flow behavior of bidisperse aqueous silica suspensions has been studied at different electrolyte concentrations as a function of shear rate, total volume fraction of the particles, and volume ratio of small to large particles. It is shown that the range of the electrostatic repulsion plays an important role in determining the viscosity of the suspension. Binary mixtures of particles of longer range repulsive forces showed higher viscosities than the suspensions of shorter range electrostatic interactions. Bimodal suspensions of long-range interactions showed non-Newtonian behavior over wider ranges of shear due to the deformation of the ionic cloud around the particles, which is larger in these systems. The viscosity of bimodal suspensions used in this study was scaled with respect to the viscosity of the related monosized systems and the viscosity of one bimodal suspension at a fixed total volume fraction of the particles, employing our earlier scaling method. The model normalizes the effect of colloidal forces by introducing a scaling factor that collapses the data into a single curve for bimodal suspensions of a particular size ratio, and it is shown that the model is valid for systems with both short-range and long-range repulsive forces. Copyright 1999 Academic Press.

Adsorption Mechanism(s) of Poly(Ethylene Oxide) on Oxide Surfaces

Adsorption of poly(ethylene oxide) (PEO) on various oxides and the flocculation behavior of PEO-coated particles was investigated to elucidate the polymer adsorption mechanism(s). It was determined that strong Bronsted acid sites on the surface interact with the ether oxygen of PEO, a Lewis base, to induce adsorption and subsequent flocculation of the substrate particles. In general highly acidic oxides of the type MO3, M2O5, and MO2 are expected to adsorb and flocculate with PEO. Accordingly, MoO3, V2O5, and SiO2 were found to strongly adsorb PEO and exhibit flocculation. On the other hand, no significant adsorption was observed on oxides with a point of zero charge (pzc) greater than that of silica such as TiO2, Fe2O3, Al2O3- and MgO thereby indicating specificity of PEO-surface bonding site interactions. The other possible mechanisms of PEO adsorption such as complexation with adsorbed ions and electrostatic interactions with a positively charged surface were found not to play a major role in the PEO adsorption onto oxide particles. In this investigation, the adsorption and flocculation behavior of oxides with PEO and the underlying adsorption mechanism(s) are discussed. Copyright 1997 Academic Press. Copyright 1997Academic Press