Tuning friction and rheology at material-nanoparticle-liquid interfaces with an external electric field.

Nanoparticles, both with and without polymeric surface coatings, dispersed in solutions are in common use as rheological and friction modifiers. The surface functionalization of the nanoparticles therefore provides a unique opportunity for active electro-tunable control of their flow in liquid. We report the use of electrophoretic forces to tune friction and rheology at material-nanoparticle-liquid interfaces with static or low frequency (0.6 – 50 mHz) electric fields. Negatively charged TiO2 or positively charged Al2O3 nanoparticles suspended in water were repositioned relative to a planar platinum surface of a quartz crystal microbalance, which was then used to monitor friction levels. Active electro-tunable control of friction was achieved, and investigated as a function of electric field frequency. Kinetic effects corresponding to nanoparticle repositioning at the solid interface were discovered to occur at glass-like time scales. The studies also reveal that nanoparticles manipulated by electric fields can act as "cantilever-free" atomic force probes capable of “tapping mode” exploration of interfacial properties and nanoscale interactions in geometries inaccessible to optical and micromechanical probes.