Surface forces and friction between cellulose surfaces in aqueous media

Colloidal probe microscopy was employed to study normal forces and sliding friction between model cellulose surfaces in aqueous solutions. Hydrodynamic interactions must be accounted for in data analysis. Long-range interactions are governed by double layer forces, and once surfaces contact, by osmotic repulsive forces and visco-elasticity. Increasing the ionic strength decreases surface potentials and increases adhesive forces. Polyelectrolytes cause strong steric repulsion at high surface coverage, where interactions are sensitive to probe velocity. Polymer bridging occurs at low coverage. Regardless of scan size, friction exhibits irregular stick-slip behavior related to surface roughness. At small scan sizes (on the order of 10 nm) the lateral force decreases with increasing load. Above a critical scan size of about 100 nm corresponding to the average size of asperities on one of the model surfaces-lateral forces are independent of scan size, but depend on the load. Hydrodynamic forces contribute little to friction. Small amounts of high molecular weight, water-soluble polymers significantly decrease sliding friction between cellulose surfaces.