Contact Stiffness and Damping of Liquid Films in Dynamic Atomic Force Microscopy

TL;DR

This study uses molecular dynamics simulations to analyze how liquid films' contact stiffness and damping behave under dynamic AFM, revealing phase relationships and effects of film solidification on mechanical properties.

Contribution

It provides new insights into the relationship between solvation forces, stiffness, and damping in liquid films during dynamic AFM, especially regarding solidified monolayers.

Findings

Contact stiffness and damping oscillate in phase with solvation forces.

Solidified monolayers show increased stiffness and decreased damping upon compression.

Mechanical excitation enhances molecular diffusion in solidified films.

Abstract

Small-amplitude dynamic atomic force microscopy (dynamic-AFM) in a simple nonpolar liquid was studied through molecular dynamics simulations. We find that within linear dynamics regime, the contact stiffness and damping of the confined film exhibit the similar solvation force oscillations, and they are generally out-of-phase. For the solidified film with integer monolayer thickness, further compression of the film before layering transition leads to higher stiffness and lower damping. We find that molecular diffusion in the solidified film was nevertheless enhanced due to the mechanical excitation of AFM tip.