Spontaneous Charging and Crystallization of Water Droplets in Oil

TL;DR

This study models the spontaneous charging and crystallization of water droplets in oil, revealing how curvature and ion partitioning influence these phenomena and aligning predictions with experimental observations.

Contribution

It introduces a numerical Poisson-Boltzmann approach to analyze ion distribution, surface charge, and crystallization in water droplets within oil, considering curvature effects.

Findings

Surface charge density is affected by droplet curvature and ion partitioning.

Crystallization conditions are enhanced compared to planar interface models.

Results align better with recent experimental data.

Abstract

We study the spontaneous charging and the crystallization of spherical micron-sized water-droplets dispersed in oil by numerically solving, within a Poisson-Boltzmann theory in the geometry of a spherical cell, for the density profiles of the cations and anions in the system. We take into account screening, ionic Born self-energy differences between oil and water, and partitioning of ions over the two media. We find that the surface charge density of the droplet as induced by the ion partitioning is significantly affected by the droplet curvature and by the finite density of the droplets. We also find that the salt concentration and the dielectric constant regime in which crystallization of the water droplets is predicted is enhanced substantially compared to results based on the planar oil-water interface, thereby improving quantitative agreement with recent experiments.