The effect of surface heterogeneities on colloidal forces

Understanding the interaction forces that can act between particles and surfaces continues to be the focus of an extensive amount of research. While direct measurements of the force between molecularly smooth surfaces generally show good agreement with traditional models, experiments performed with more natural particles and surfaces frequently show substantial discrepancies. One well-known example is the deposition rate of particles flowing through a packed bed; numerous experimental studies have found significant deposition occurring when essentially none was predicted. Many researchers have suggested that the cause of such discrepancies is heterogeneities on the interacting surfaces, either in the form of surface roughness or a non-uniform surface charge density. Although accounting for such factors requires some simplifying assumptions about the nature of the heterogeneity (e.g. that the roughness is in the form of hemispherical asperities), the models which have been developed have indeed provided much better agreement with measured results. This article is a review of some of the work that has been performed to understand the effect of surface roughness and a non-uniform surface charge density on colloidal forces. Long range, short range and particle adhesion forces are included and the relative advantages and limitations of the computational approaches are discussed where appropriate. It is shown that although various modeling strategies have been used, the predicted trends are generally in qualitative agreement. Also presented are applications of these models to the prediction of particle flocculation and deposition rates.