(104h) Role of Surface Microstructure and Elasticity in Promoting Lubricated Contact and Adhesion

Despite its importance in many applications and processes, a complete and unified view on how deformable nano- and micro-scale asperities influence hydrodynamic interactions has yet to be reached. In particular, the competing effects of surface structure and elastic compliance can be expected to become more dominant with soft materials when the length scale of the asperities or textures become comparable to that of the fluid flow. By combining experiments with the surface forces apparatus (SFA) with experiments performed with a custom-built peeling instrument¹, we characterized and modeled the hydrodynamic forces between a smooth plane and lyophilic surfaces decorated with hexagonal arrays of isolated (holes) and interconnected (pillars) microstructures in a viscous silicone oil. We have also investigated how elastic compliance competes with drainage through asperities in the viscous forces during both approach and retraction, especially in the peeling configuration¹. For all the microstructures studied, the periodicity of the surface features was much larger than the separation range of our measurements, hence formed a thin channel geometry.²

In this talk, the approach and separation forces measured for both normal and peeling configurations with different microstructures and elasticity will be compared to predictions for existing theoretical boundary condition models for smooth surfaces (Reynolds theory), interfacial slip and a shifted no-slip plane. We also highlight the contribution of surface deformation³ and compare our peeling results to a model based on hydrodynamics and bending beam. Finally we determine the surface, mechanical, and fluid properties that are necessary for surface structures to enhance adhesion under flooded conditions. In particular we highlight the importance of van der Waals forces and viscoelasticity. We show that in absence of these two effects the presence of surface structure does not lead to enhancement in adhesion (and in many cases reduces the adhesion), even with highly compliant surfaces.

1. Dhong, C. and J. Fréchette, Coupled effects of applied load and surface structure on the viscous forces during peeling. Soft matter, 2015. 11(10): p. 1901-1910.

2. Pilkington, G.A., R. Gupta, and J. Fréchette, Scaling Hydrodynamic Boundary Conditions of Microstructured Surfaces in the Thin Channel Limit. Langmuir, 2016. 32(10): p. 2360-2368.

3. Wang, Y., C. Dhong, and J. Frechette, Out-of-Contact Elastohydrodynamic Deformation due to Lubrication Forces. Physical review letters, 2015. 115(24): p. 248302.