Amplitude modulation atomic force microscopy, is acoustic driving in liquid quantitatively reliable?

Area of Science:

• Surface Science
• Nanotechnology
• Physical Chemistry

Background:

• Measuring tip-sample interaction forces in dynamic atomic force microscopy (AFM) in fluids is challenging due to viscous damping and fluid-mediated driving forces.
• Acoustic excitation of the cantilever is commonly used but its quantitative reliability in liquids is often questioned.

Purpose of the Study:

• To present quantitative measurements of tip-sample interactions in aqueous electrolytes.
• To evaluate the reliability of different analysis approaches for acoustic excitation in liquids.

Main Methods:

• Measurements of conservative (DLVO, hydration) and dissipative forces using AFM with varying tip radii (20 nm to 1 μm).
• Analysis using a simple harmonic oscillator model, continuous beam theory with fluid-mediated excitation, and thermal noise spectroscopy (TNS).
• Comparison of experimental results with theoretical predictions.

Main Results:

• Consistent conservative forces were obtained across all three analysis approaches, with deviations less than 40%.
• DLVO forces agreed within 5% of theoretical expectations for all methods.
• Accurate dissipative force measurements (within 15% of predictions) require TNS or continuous beam theory accounting for fluid-mediated driving.

Conclusions:

• Acoustic driving in liquid is quantitatively reliable for measuring tip-sample forces when appropriate analysis methods are employed.
• The study validates the use of TNS and continuous beam theory for accurate dissipative force measurements in fluidic AFM.