Interfacial Effects during Phase Change in Multiple Levitated Tetrahydrofuran Hydrate Droplets

TL;DR

This study investigates the interfacial effects during phase change in multiple levitated tetrahydrofuran hydrate droplets, revealing mechanisms of nucleation, opacity loss, and internal convection that influence hydrate formation and droplet behavior.

Contribution

First detailed analysis of simultaneous nucleation and phase change mechanisms in multiple levitated hydrate droplets using digital and infrared imaging.

Findings

Nucleation initiates at the aqueous-air interface with pseudo-heterogeneous mechanisms.

Droplets exhibit optical clarity loss due to tiny air bubbles trapped during rapid hydrate growth.

Droplets develop internal convection and spin due to thermal gradients during melting.

Abstract

In this study, using direct digital and infrared imaging techniques, the freezing of up to three simultaneous THF hydrate droplets was investigated for the first time. Nucleation was initiated at the aqueous solution-air interface. Two pseudo-heterogeneous mechanisms created additional nucleation interfaces: one from cavitation effects entraining microbubbles and another from subvisible ice particles, also called hydrate nucleating particles (HNPs), impacting the droplet surface. For systems containing droplets in both the second and third positions, nucleation was statistically simultaneous between all droplets. This effect may have been caused by the high liquid-solid interfacial pressures that developed at nucleation, causing some cracking in the initial hydrate shell around the droplet and releasing additional HNPs (now of hydrate) into the air. During crystallization, the THF hydrate droplets developed a completely white opacity, termed optical clarity loss or OCL. It was suggested that high hydrate growth rates within the droplet resulted in the capture of tiny air bubbles within the solid phase. In turn, light refraction through many smaller bubbles resulted in the OCL. These bubbles created structural inhomogeneities, which may explain how the volumetric expansion of the droplets upon complete solidification was 23.6% compared with 7.4% in pure, stationary THF hydrate systems. Finally, the thermal gradient that developed between the top and bottom of the droplet during melting resulted in a surface tension gradient along the air-liquid interface. In turn, convective cells developed within the droplet, causing it to spin rapidly about the horizontal axis.