are dependent on the interactions between colloidal particles
The van der Waals and electrostatic interaction energies beor particles and macroscopic surfaces. Traditionally, colloitween a single particle and a flat plate were measured using the dal interactions have been described by the DLVO theory optical technique of total internal reflection microscopy (TIRM). (1,2) in which the total interaction energy between two The particles used were 15-mm-diameter polystyrene latex spheres surfaces is calculated as the sum of the repulsive electrostatic and the flat plate was a polished BK-7 glass slide. The measuredouble-layer interaction and the attractive van der Waals ments were performed in aqueous solutions of ionic strength ranginteraction.
ing from 3 to 7 mM, and the particle-plate separation distances
The original DLVO theory assumed that the interacting ranged from approximately 25 to 300 nm. During measurement, surfaces were perfectly smooth. Over the years, various rethe particle was trapped in a secondary energy well formed by the repulsive electrostatic and attractive van der Waals forces; searchers have developed theoretical models to account for however, the particle was free to undergo Brownian motion at the presence of roughness inherent on essentially all surall times. These measurements, which capitalize on the extreme faces. Good reviews of the previous work in this area are sensitivity of the TIRM technique, are believed to be the first direct given by Elimelech et al. (3) and Suresh and Walz (4). measurement of the van der Waals interaction energy in aqueous Nearly all prior work indicates that surface roughness greatly solutions at separation distances where retardation effects are subreduces the potential energy barrier to particle flocculation stantial. Comparison of the measured energy wells with predicand deposition, in qualitative agreement with experimental tions made with traditional energy equations produced only fair observations. agreement; specifically, the measured well depths were consistently Direct experimental measurement of the effect of lower than predicted. However, when the measured results were roughness on the long-range van der Waals and electrostatic compared with predictions made using the recent model of L.
interactions has been extremely limited. In fact, the objective