A novel coarsening mechanism of droplets in immiscible fluid mixtures

TL;DR

This paper introduces a new hydrodynamic mechanism driven by Marangoni forces for droplet coarsening in immiscible liquids, challenging the traditional Brownian coagulation explanation.

Contribution

It reveals that droplet motion during coarsening is hydrodynamically driven by composition gradients, not random thermal motion, fundamentally changing the understanding of the process.

Findings

Hydrodynamic Marangoni forces dominate droplet motion.

Coarsening is driven by composition correlation among droplets.

Contradicts the Brownian coagulation mechanism.

Abstract

In our daily lives, after shaking a salad dressing, we see the coarsening of oil droplets suspended in vinegar. Such a demixing process is observed everywhere in nature and also of technological importance. For a case of high droplet density, domain coarsening proceeds with interdroplet collisions and the resulting coalescence. This phenomenon has been explained primarily by the so-called Brownian coagulation mechanism: stochastic thermal forces exerted by molecules induce random motion of individual droplets, causing accidental collisions and subsequent interface-tension driven coalescence. Contrary to this, we demonstrate that the droplet motion is not random, but hydrodynamically driven by the composition Marangoni force due to an interfacial tension gradient produced in each droplet as a consequence of composition correlation among droplets. This alters our physical understanding of droplet coarsening in immiscible liquid mixtures on a fundamental level.