  The use of electrophoretic forces to tune friction at
material-nanoparticle-liquid interfaces with static or low frequency (0.6-50
mHz) electric fields is reported for the first time. External electric fields
were employed to reposition negatively charged TiO2 or positively charged Al2O3
nanoparticles suspended in water in directions perpendicular to a planar
platinum surface of a quartz crystal microbalance, which was then used to
monitor frictional shear forces at the interface. Active electro-tunable
control of friction has been demonstrated for both TiO2 and Al2O3 suspensions.
For TiO2 suspensions, significant drops in frictional shear forces, not
observed for Al2O3, were likely attributed to the presence of molecularly thin
interstitial water layers remaining in regions between the TiO2 particles and
the substrate. Timescales associated with motion of nanoparticles in directions
perpendicular to the surface were also investigated by varying the frequency of
the external electric field, and were determined to be similar to those of
glass-like or polymeric materials. Overall, the studies reveal that
nanoparticles actively driven 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.
