Long-range hydrodynamic forces in liquid FM-AFM.

Area of Science:

• Physics
• Physical Chemistry
• Materials Science

Background:

• Atomic Force Microscopy (AFM) is a powerful tool for nanoscale imaging and force measurements.
• Frequency-Modulation AFM (FM-AFM) enhances sensitivity by operating in a dynamic mode.
• Understanding hydrodynamic effects is crucial for accurate FM-AFM operation in liquid environments.

Purpose of the Study:

• To investigate and quantify the influence of hydrodynamic forces on FM-AFM measurements in liquid.
• To establish theoretical frameworks for extracting hydrodynamic interaction stiffness and damping from experimental data.
• To experimentally validate theoretical predictions across a wide range of probe-surface distances.

Main Methods:

• Derivation of theoretical equations relating frequency shift and excitation amplitude to hydrodynamic interaction stiffness (k_int) and damping (β_int).
• Development and execution of specialized FM-AFM experiments in water.
• Systematic measurement of k_int and β_int as a function of probe-surface distance (up to 200 µm).

Main Results:

• Experimental data for k_int closely matches theoretical predictions, indicating unsteady hydrodynamic forces are dominant at both short and long distances.
• Measured β_int is minimal at large distances and increases significantly, diverging as the probe approaches the surface.
• Observed damping behavior aligns with the classical Reynolds sphere model at short distances.

Conclusions:

• Hydrodynamic forces play a critical, distance-dependent role in FM-AFM measurements in liquids.
• The study provides a robust method for characterizing hydrodynamic interactions in FM-AFM.
• The findings contribute to more precise interpretation of FM-AFM data in fluidic conditions.