(27c) Influence of Surface Roughness on Particle-Substrate Interactions: Double Layer Repulsion, Van Der Waals Attraction, and Depletion Interaction

The interaction of colloidal particles with each other and with solid substrates has long been studied, because it determines dispersion stability and deposition kinetics; it also holds the key to directing any form of colloidal self-assembly. For particles in dilute dispersions interacting across simple electrolytes at low to moderate ionic strength, the relevant interaction forces are considered reasonably well understood; and still, discrepancies between theory and experiment are observed routinely, and theoretical predictions rarely prove quantitatively accurate. Neglected effects of surface roughness are often blamed for these problems, but efforts to account for roughness and test the proposed modeling strategies have been limited. One difficulty in assessing the influence of surface topography lies in varying surface roughness in a controlled way without concomitantly changing the surface chemistry. In our present study, the particle-substrate interaction of microparticles with tunable surface roughness at fixed surface chemical composition was probed using Total Internal Reflection Microscopy (TIRM). These measurements reveal a simultaneous enhancement of the electric double layer interaction and weakening of the van der Waals interaction caused by the surface roughness. We further used small particles with negligible scattering intensity in the evanescent wave to induce depletion interaction between the probe particle and the substrate. At a fixed size and concentration of the depletant particles we studied the effect of probe particle roughness up to roughness amplitudes comparable to the depletant size. As one might expect, the depletion interaction gets weaker with increased particle roughness, but to our knowledge, ours is the first quantitative study of this roughness effect, which is analogous to the effect of molecular-scale roughness on the solvent-mediated “structural forces” reported from observations with the surface forces apparatus.