Lubrication forces in air and accommodation coefficient measured by a thermal damping method using an atomic force...

Area of Science:

• Physics
• Materials Science
• Surface Science

Background:

• Understanding gas-surface interactions is crucial for micro/nanoscale devices.
• Atomic Force Microscopy (AFM) offers high-resolution force measurements.
• Characterizing gas flow regimes (slip, transition, molecular) is essential for modeling.

Purpose of the Study:

• To measure damping and static forces on a sphere near a flat plate using AFM.
• To determine gas-surface interaction parameters like slip length and momentum accommodation coefficient.
• To investigate gas flow regimes from slip to molecular flow at a single pressure.

Main Methods:

• Utilized thermally driven oscillations of an Atomic Force Microscope (AFM) cantilever.
• Varied the proximity between a sphere and a flat plate to alter the Knudsen number (Kn).
• Analyzed damping and static forces in different gas flow regimes.

Main Results:

• Measured a combined slip length of 250 nm ± 100 nm for methylated glass in ambient air at 1 atm.
• Determined the tangential momentum accommodation coefficient (σ) to be 0.77 ± 0.24.
• Observed deviations from linearity in inverse damping at small separations (Kn > 0.4), agreeing with the Vinogradova formula.

Conclusions:

• AFM is effective for characterizing gas-surface interactions across multiple flow regimes.
• Quantified slip length and momentum accommodation for specific methylated glass surfaces.
• Validated theoretical models for gas flow in the transition and molecular regimes.