Electrically Tunable Friction through Surface Adsorption Layer Restructuring

Electric-potential-controlled friction, which manipulates the frictional response of lubricants via an applied potential, offers the possibility of on-demand lubrication. Conventional understanding suggests that the applied potential influences the adsorption of surfactant ions on rubbing surfaces, thereby altering friction. This study investigates the effect of applied potential on the tribological behavior of sodium dodecyl sulfate (SDS) aqueous solutions in steel-steel contacts through experiments and molecular simulations. It is shown that SDS, as an anionic surfactant, readily forms hemicylindrical surface micelles due to electrostatic and hydrophobic interactions, achieving high coverage even at low concentrations. Consequently, the adsorbed Na⁺ counterions are more responsive to the applied potential than the SDS anions. Contrary to the common belief, friction in steel-steel contacts is governed by Na⁺ concentration through its role in manipulating the structures of the adsorbed SDS aggregates. A critical Na⁺ concentration─achieved either through concentrated SDS solutions or added sodium salt─is required for friction to increase with increasingly negative potential. This friction increase can be attributed to a transition from hemicylindrical to hemispherical surface micelles. This work underscores the competing roles of electrostatic and hydrophobic interactions in surfactant lubrication, suggesting that an effective electro-responsive additive must balance these interactions to enable potential-driven modulation. These findings provide key insights for the design of smart lubricants with potential-tunable friction properties.