Nanoscale surface roughness effect on Derjaguin-Landau-Verwey-Overbeek forces

In this work we investigated the effects of nanoscale roughness on Derjaguin-Landau-Verwey-Overbeek (DLVO) forces using the proximity force approximation. To that end, the force between a gold-coated sphere and a plate, both immersed in pure and salt-Added [NaCl]=0.01M ethanol, was measured as a function of surface roughness with atomic force microscopy. The force data were compared with the predictions of DLVO theory with and without incorporating the contribution of surface roughness. It was observed that the surface roughness does not modify the qualitative behavior of the force-displacement curve but rather has a secondary influence by altering the values of the free parameters of the DLVO theory, namely, the Debye length and the electrostatic potential of the interacting bodies. In fact, it was found that the repulsive component of the force in pure ethanol increased with increasing roughness. These observations coincide with a positive correlation of the absolute value of the surface potentials with increasing surface roughness, suggesting that roughness effectively enhances the electric field near the surfaces, resulting in stronger electrostatic forces. For the salt-Added solution, the contributions of the electrostatic forces are heavily suppressed, and the Casimir contribution plays the dominant role. Finally, we have shown that failure to incorporate the complete optical properties of the interacting metal bodies yields unphysical results for the free parameters of the electrostatic double-layer force.