Evaluating the Laplace pressure of water nanodroplets from simulations

TL;DR

This study uses molecular simulations to analyze the microscopic pressure distribution in water nanodroplets, confirming classical predictions even at nanoscale sizes.

Contribution

It introduces a modified coarse-graining method for calculating the microscopic pressure tensor in rigid water models.

Findings

Pressure becomes isotropic and constant beneath a few molecular layers.

Pressure dependence on droplet size aligns with Young-Laplace equation.

Nanodroplets exhibit classical pressure behavior despite their small size.

Abstract

We calculate the components of the microscopic pressure tensor as a function of radial distance r from the centre of a spherical water droplet, modelled using the TIP4P/2005 potential. To do so, we modify a coarse-graining method for calculating the microscopic pressure [T. Ikeshoji, B. Hafskjold, and H. Furuholt, Mol. Simul. 29, 101 (2003)] in order to apply it to a rigid molecular model of water. As test cases, we study nanodroplets ranging in size from 776 to 2880 molecules at 220 K. Beneath a surface region comprising approximately two molecular layers, the pressure tensor becomes approximately isotropic and constant with r. We find that the dependence of the pressure on droplet radius is that expected from the Young-Laplace equation, despite the small size of the droplets.