Pressure effects on the shearing of a colloidal thin film

The pressure effect in the frictional behaviour of a colloidal suspension of over-based calcium carbonate stabilized in pure dodecane has been investigated. With a surface force apparatus, the authors have observed the squeeze effect and the frictional resistance of the suspension using sphere- and plane-geometry on a nanometric scale and for low contact pressure (10 4 to 10 6 Pa). With tribometers, which can support heavy loading, they have analysed the high pressure domain (10 4 to 10 9 Pa). After a critical pressure, evaluated at 10 6 Pa, the colloidal film does not flow in the contact of the solids, but forms a compacted 'mattress' sliding on the plane surface, and squeezing a hydrocarbon layer. The mean shear strength of the interface, tau , can then be expressed by a relation of the form tau = tau 0 + alpha p, where p is the mean contact pressure. The meaning of tau 0 and alpha is discussed.