Sliding friction between cellulose and silica surfaces

An atomic force microscope was used to measure kinetic friction forces between a colloidal cellulose sphere and different planar surfaces in air. The planar surfaces were bare silica, silica modified with a monolayer of alkyl ketene dimer (AKD), a common additive used for paper sizing, and silica with surface grafted terminal methyl groups. Friction was measured as a function of the effective load resulting from adhesive interactions between the cellulose sphere and the planar surface. Friction in the regime of low effective loads was shown to be a linear function of the load, and the friction coefficients determined from the slope of these curves were 0.26, 0.07, and 0.021 for the cellulose-silica, cellulose-AKD, and cellulose-methylated silica systems, respectively. The ratio between friction coefficients for the two first systems agrees well with that reported for paper in the presence and absence of AKD sizing agents, respectively. In the high effective load regime, deviations from linearity between measured friction and load were observed. This is an indication that we have single asperity contact, at least at high loads. These results were analyzed in terms of a contact mechanics transition equation, By making assumptions on the moduli of the materials, the interfacial shear strength tau and the contact radius ao were calculated. The results obtained through this analysis provide an important insight into the relationship between friction and chemical surface properties.