Direct measurement of molecular stiffness and damping in confined water layers

TL;DR

This study uses atomic force microscopy to directly measure the stiffness and damping of confined water layers at the molecular scale, revealing oscillatory behavior and the influence of solvation effects on dynamics.

Contribution

It provides the first direct, linear measurements of molecular stiffness and damping in confined water, highlighting the role of solvation and cavity commensurability in system dynamics.

Findings

Observed up to 7 water layers separated by ~2.56 Å.

Found relaxation times increase as confinement reduces.

Dynamics influenced more by solvation effects than pressure.

Abstract

We present {\em direct} and {\em linear} measurements of the normal stiffness and damping of a confined, few molecule thick water layer. The measurements were obtained by use of a small amplitude (0.36 $\textrm{\AA}$), off-resonance Atomic Force Microscopy (AFM) technique. We measured stiffness and damping oscillations revealing up to 7 layers separated by 2.56 $\pm$ 0.20 $\textrm{\AA}$. Relaxation times could also be calculated and were found to indicate a significant slow-down of the dynamics of the system as the confining separation was reduced. We found that the dynamics of the system is determined not only by the interfacial pressure, but more significantly by solvation effects which depend on the exact separation of tip and surface. Thus ` solidification\rq seems to not be merely a result of pressure and confinement, but depends strongly on how commensurate the confining cavity is with the molecule size. We were able to model the results by starting from the simple assumption that the relaxation time depends linearly on the film stiffness.