The Effect of Nanoscale Roughness on Long Range Interaction Forces

A model was developed to calculate the long range van der Waals and electrostatic energies between a rough colloidal particle and a smooth solid plate in aqueous solutions. The particle roughness was modeled as hemispherical asperities distributed uniformly over the surface. Because of the assumption of additive potentials used in calculating the electrostatic force, the model is most accurate when the particle/plate separation is larger than several Debye screening lengths, such as near the location of the secondary minimum. The model predicts that such roughness reduces the depth of the secondary minimum and pushes it to larger separation distances. The model also predicts that the height of the primary energy barrier, which controls the dispersion stability, is substantially lowered by the asperities.Experimental validation of the model was achieved by measuring the potential energy profile between individual, 15µm diameter polystyrene latex spheres and a smooth glass plate around the location of the secondary energy minimum using the optical technique of total internal reflection microscopy (TIRM). When compared to predictions made assuming perfectly smooth surfaces, the measured well depths are consistently found to be lower than expected. Excellent agreement can be achieved, however, by adding asperities with a height of order 25nm to the particle surface. These measurements are some of the first direct measurements of the effect of roughness on interaction energies.