Lubrication by organized soft matter

The AFM-colloidal probe technique has been used to explore surface interactions and friction forces between polyelectrolyte-coated surfaces immersed in aqueous solutions in the absence and presence of surfactant. It is found that the nature of the load bearing forces is decisive for the lubricating properties of the layers. Low frictional forces are obtained when the load is carried by a force that allows the interfacial layer to have a high fluidity, whereas attractive surface forces most often increase the friction. Highly charged polyelectrolytes and oppositely charged surfactants associate in bulk solution to form complexes with a well-defined internal structure. At the surface, similarly structured polyelectrolyte-surfactant layers are spontaneously formed. Such layers have a high load bearing capacity, and the friction coefficient is very low as long as the integrity of the layer remains intact. Interestingly, when the load is increased step-wise the friction force as a function of load displays some sharp peaks, which are identified as being due to structural rearrangements in the polyelectrolyte-surfactant layer. On unloading very low frictional forces are obtained despite the presence of an adhesion between the layers. To maintain the favorable lubricating properties it is essential to have surfactants present in solution, whereas, after the initial adsorption step, there is no need to have the polyelectrolyte present in the bulk. The reason for this is the essentially irreversible adsorption of the polyelectrolyte.