Polymer Brush-Enhanced Extraction and Spreading of Oil from Lubricating Greases

Grease lubrication is among the oldest and most widely employed strategies for reducing friction in tribological contacts. Nevertheless, important microscopic mechanisms that govern the release and transport of lubricants in, say, rolling bearings remain poorly understood. Here, we show how the release of lubricating oil from grease reservoirs and its subsequent spreading towards the rolling contact can be controlled by different treatments of the underlying surface. We demonstrate that smooth, flat oxide surfaces, where oil films are stabilized exclusively by molecular wetting forces, are unable to extract and spread appreciable amounts of oil. In contrast, functionalizing the surface by topographic roughening and/or by coatings of swelling oleophilic polymer brushes can enhance the oil extraction by over two orders of magnitude. Specifically, polymer brushes with an intrinsic phase transition are shown to enable an adaptive lubrication scheme, where the amount of lubricant provided increases rapidly when the temperature increases beyond the melting temperature of the surface-grafted polymer coating.