Computational Simulations of Solvation Force and Squeeze Out of Dodecane Chain Molecules in Atomic Force Microscopy

TL;DR

This study uses all-atom molecular dynamics simulations to explore solvation force oscillations and squeeze out mechanisms of dodecane molecules confined between an AFM tip and a substrate, revealing insights into nanoconfined liquid behavior.

Contribution

It provides detailed molecular-level insights into solvation force oscillations and squeeze out mechanisms of linear chain molecules under confinement, using all-atom AFM simulations.

Findings

Solvation force oscillations linked to layering transitions.

Molecular permeation and edge squeeze out promote layering.

Confinement slows dynamics but maintains diffusive motion.

Abstract

There is a growing interest since the 1990s to understand the squeezing and shear behaviors of liquid films at nanometer scale by the atomic force microscope (AFM) measurement. We carry out all-atom contact-mode AFM simulations in a liquid-vapor molecular dynamics ensemble to investigate the solvation force oscillation and squeeze out mechanisms of a confined linear dodecane fluid between a gold AFM tip and a mica substrate. Solvation force oscillations are found to be associated with the layering transition of liquid film and unstable jumps of AFM tip position. Detailed structural analyses and molecular animations show that the local permeation of chain molecules and the squeeze out of molecules near the edge of contact promote the layering transition under compression. The confinement-induced slow down dynamics is manifested by the decrease in diffusivity and increase in rotational relaxation times. However, the persistent diffusive behavior of dodecane chain molecules even in the single-monolayer film is attributed to the chain sliding motions, given the fact that substantial vacancy space exists in the film due to thermal fluctuations.