Tuning friction and slip at solid-nanoparticle suspension interfaces by electric fields.*

Pathways to achieve climate stabilization consistently require a diverse mix of technologies, with energy efficiency being the largest contributor, ranging from 35-40% in virtually all proposed scenarios. Reductions in frictional energy losses and improved lubrication methodologies are key to such energy savings. Nanoparticles in aqueous suspensions hold particular promise for such purposes. We report an experimental Quartz Crystal Microbalance (QCM) study of tuning interfacial friction and slip lengths for aqueous suspensions of ceramic (Al₂O₃, TiO₂ and SiO₂) nanoparticles on planar platinum surfaces by external electric fields. Attraction and retraction of particles perpendicular to the surface by means of an externally applied fields resulted in increased and decreased interfacial friction levels and slip lengths. The variation was observed to be non-monotonic, with a profile attributed to the physical properties of interstitial water layers present between the nanoparticles and the platinum substrate. The results are compared and contrasted with macroscale friction measurements performed on the same materials.