We report an experimental Quartz Crystal Microbalance (QCM) study of tuning interfacial friction and slip lengths for aqueous suspensions of TiO2 and Al2O3 nanoparticles on planar platinum surfaces by external electric fields. Data were analyzed within theoretical frameworks that incorporate slippage at the QCM surface electrode or alternatively at the surface of adsorbed particles, yielding values for the slip lengths between 0 and 30 nm. Measurements were performed for negatively charged TiO2 and positively charged Al2O3 nanoparticles in both the absence and presence of external electric fields. Without the field the slip lengths inferred for the TiO2 suspensions were higher than those for the Al2O3 suspensions, a result that was consistent with contact angle measurements also performed on the samples. Attraction and retraction of particles perpendicular to the surface by means of an externally applied field 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.

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A quartz crystal microbalance (QCM) technique has been employed to tune friction at liquid-solid interfaces with tribotronic methods employing externally applied electric fields in both nanoparticle suspensions and ionic liquid systems. The setup consists of a QCM immersed in liquid containing electrically charged constituents whose sensing electrode faces a nearby counter electrode. An electric field perpendicular to the QCM surface is created when a potential is applied between the two electrodes, which allows the charged constituents in the surrounding liquid to be repositioned. QCM measurements are able to detect differences in friction under various field conditions, and thus detectably tune the friction in both nanoparticle and ionic liquid systems. The versatility and simplicity of QCM friction measurements render it an ideal tool for the rapidly expanding research area of tribotronics.

We report a study of the response of a Quartz Crystal Microbalance (QCM) to rubbing contacts in air, water and aqueous suspensions of 40 nm TiO 2 nanoparticles. Measurements were performed with a contact comprised of 3 close-packed 304SS ball bearings situated symmetrically about the center of a 304SS QCM electrode with 2 nm rms roughness. Two continuum methods were employed to infer macroscale friction coefficients µ employing QCM nanotribological data recorded in the Cattaneo-Mindlin (CM) slip regime at vibrational amplitudes that varied between 1 and 17 nm. The "slope" Method 1 involved sweeps of the QCM amplitude of vibration as ball bearings were held in continuous contact with the oscillating electrode. The "contact" Method 2 obtained µ by analyzing the shifts in frequency and bandwidth that occur at a fixed u o to solve for µ. when ball bearings were brought in and out of contact with the QCM's electrode. The results for dry and water lubricated contacts compared favorably with macroscale friction coefficients reported in the literature. The model failed to adequately describe contacts lubricated with the NP suspension, but its continuum nature did not appear to be the dominant factor underlying failure. The failure was more likely attributable to either a lack of a CM slip regime when NP were present at the interface and/or the fact that the amplitude of vibration was close in size to the individual NP contacting regions, in violation of a key underlying assumption of the model.