| Abstract
| - Using a surface forces apparatus (SFA) and an atomic force microscope (AFM) we have studied the effectsof surface roughness (root-mean-square (RMS) roughness between 0.3 and 220 nm) on the “contact mechanics”,which describes the deformations and loading and unloading adhesion forces, of various polymeric surfaces.For randomly rough, moderately stiff, elastomeric surfaces, the force−distance curves on approach andseparation are nearly reversible and almost perfectly exponentially repulsive, with an adhesion on separationthat decreases only slightly with increasing RMS. Additionally, the magnitude of the preload force is seen toplay a large role in determining the measured adhesion. The exponential repulsion likely arises from the localcompressions (fine-grained nano- or submicron-scale deformations) of the surface asperities. The resultingcharacteristic decay lengths of the repulsion scale with the RMS roughness and correlate very well with asimple finite element method (FEM) analysis based on actual AFM topographical images of the surfaces. For“patterned” surfaces, with a nonrandom terraced structure, no similar exponential repulsion is observed,suggesting that asperity height variability or random roughness is required for the exponential behavior.However, the adhesion force or energy between two “patterned” surfaces fell off dramatically and roughlyexponentially as the RMS increased, likely owing to a significant decrease in the contact area which in turndetermines their adhesion. For both types of rough surfaces, random and patterned, the coarse-grained (global,meso- or macroscopic) deformations of the initially curved surfaces appear to be Hertzian.
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