  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.
