Droplet Size Distribution in Emulsions

TL;DR

This paper investigates droplet size distribution in oil-in-water emulsions stabilized by surfactants, revealing a log-normal distribution resulting from random breakup, and identifies the key scaling law based on capillary number rather than flow Reynolds number.

Contribution

It demonstrates that droplet size distribution in high-shear emulsification follows a log-normal pattern and introduces a capillary number scaling law, contrasting with turbulent emulsification theories.

Findings

Droplet size distribution is log-normal.

Size does not scale with Reynolds number.

Size scales with capillary number.

Abstract

The droplet size in emulsions is known to affect the rheological properties and plays a crucial role in the many applications of emulsions. Despite its importance, the underlying mechanisms governing droplet size in emulsification remain poorly understood. We investigate the average drop size and size distribution upon emulsification with a high-shear mixer for model oil-in-water emulsions stabilized by a surfactant. The size distribution is found to be a log-normal distribution, resulting from the repetitive random breakup of drops. High-shear emulsification, the usual way of making emulsions, is therefore found to be very different from turbulent emulsification given by the Kolmogorov-Hinze theory for which power-law distributions of the drop size are expected. In agreement with this, the mean droplet size does not follow a scaling with the Reynolds number of the emulsification flow, but rather a capillary number scaling based on the viscosity of the continuous phase.