(365e) Effect of Roughness on Interactions between Surfaces

Classical theories of surface interactions usually assume perfectly smooth planar or spherical surfaces. This talk will present analytic equations that describe the effect of surface roughness on interfacial interactions that are validated against extensive numerical calculations for self-affine elastic surfaces that have roughness on a wide range of wavelengths. First nonadhesive surfaces are considered. Surface roughness leads to an exponential repulsive force with a characteristic decay length that is of order the root mean squared (rms) surface roughness. Next the effect of van der Waals attractions is described. A simple criterion for the existence of a net adhesive force is derived and tested. The distribution of surface separations is then used to calculate contributions to the net adhesive force. Experiments are often performed using surfaces that are nominally spherical or between crossed cylinders. The effect of adding roughness to a surface with a nominal radius of curvature R will be described. A simple analytic expression captures the change in surface area with normal load for nonadhesive surfaces. At small loads the area is proportional to load and at large loads the area is described by Hertz theory. The final part of the talk will examine the effect of roughness on colloidal spheres with DLVO interactions. Roughness has little effect on the maximum repulsive barrier for contact, but greatly decreases the energy required to separate surfaces. The implications for shear thickening of dense suspensions will be discussed.