Abstract
Measurement of the damping and elastic interactions between two solids interfaces (one being the apex of a tapered probe that is attached to one tine of a quartz tuning fork while the other is a flat substrate) under relative lateral oscillatory motion are reported. The solid boundaries are separated by a nanometer sized gap, and emphasis is placed on the role played by the mesoscopic fluid trapped in between. The measurements were implemented using two new acoustic techniques that have been integrated into a tuning fork based scanning probe microscope; the whole metrology system offers sub-nanometer precision for controlling the probe’s vertical position, as well as thermal drift correction. One of the acoustic techniques, Shear-force Acoustic Near-field Microscopy (SANM), monitors the acoustic emission from the trapped fluid subjected to shear stress, while Whispering Gallery Acoustic Sensing (WGAS) monitors the oscillation amplitude of the QTF. Additionally, the elastic and damping components of the probe-fluid interaction forces are decoupled by operating the SANM in frequency modulation mode, while the eventual contact between the probe and substrate is tracked by monitoring the electrical current signal. Results involving interaction in the 0 to 5 nN regime are reported. The measurements reveal a strong correlation between the monotonic shift of the probe’s resonance frequency and the near-field acoustic emission from the trapped fluid. The SANM signal, then, accounts as one of the elastic energy dissipation channels involved in surface interactions at ambient conditions.
Highlights
The physical properties of mesoscopic fluids found adsorbed at substrates or confined between solid surfaces differ greatly from those measured in their bulk form[1,2,3]
We report the use of a new acoustic metrology tools, Shear-force Acoustic Near-field Microscopy (SANM)[15,16] and Whispering Gallery Acoustic Sensing (WGAS)[17], to characterize the dynamics of fluid films trapped between solid surfaces in relative motion
Two near-field acoustic techniques, SANM and WGAS were used to characterize the fluid trapped between a sharp probe and a flat solid surfaces at ambient conditions
Summary
The physical properties of mesoscopic fluids found adsorbed at substrates or confined between solid surfaces differ greatly from those measured in their bulk form[1,2,3]. During a probe’s trip approach, once the apex starts interacting with a mesoscopic fluid (assumed viscoelastic), the WGAS sinusoidal output signal (acoustic feedback signal in figure 2) undergoes a phase change because the probe is out of resonance.
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