Gas hydrate nucleation in acoustically levitated water droplets

Interfacial Effects during Phase Change in Multiple Levitated Tetrahydrofuran Hydrate Droplets

“…The nucleation of ice in levitated droplets has previously been estimated to have a 90% probability of occurring on the surface. , There could be a pseudo-heterogeneous mechanism at the air–liquid interface where ultrasonic waves create a concentrated acoustic pressure and cause cavitation. In turn, this promotes heterogeneous nucleation by increasing the number of microbubbles entrained at the surface. ,, Nucleation from the microbubbles at the air–liquid interface requires less work and has been reported as the most thermodynamically favorable and likely mechanism for the levitated nucleation of ice and natural gas hydrates. , This nucleation phenomenon would occur at a scale too small to measure in the current study’s system. However, the previous levitation studies that were able to provide direct evidence of the presence of this mechanism used similar ultrasonic frequencies (e.g., 34.5 or 39 kHz), indirectly indicating the likelihood of the microbubble mechanism in the present system. , In this case, the droplet surface area becomes the most influential parameter for nucleation, which can explain why the droplet with the largest volume (position 3) nucleates first.…”

“…In turn, this promotes heterogeneous nucleation by increasing the number of microbubbles entrained at the surface. ,, Nucleation from the microbubbles at the air–liquid interface requires less work and has been reported as the most thermodynamically favorable and likely mechanism for the levitated nucleation of ice and natural gas hydrates. , This nucleation phenomenon would occur at a scale too small to measure in the current study’s system. However, the previous levitation studies that were able to provide direct evidence of the presence of this mechanism used similar ultrasonic frequencies (e.g., 34.5 or 39 kHz), indirectly indicating the likelihood of the microbubble mechanism in the present system. , In this case, the droplet surface area becomes the most influential parameter for nucleation, which can explain why the droplet with the largest volume (position 3) nucleates first. However, while this mechanism may be present in the system, it does not explain why the position 1 droplet, which has a smaller surface area, nucleates at similar times to position 3.…”

“…The methods to eliminate container-driven effects on nucleation have recently been expanded to include acoustic levitation. For examining hydrate nucleation, previous investigations have been limited to single droplets contained within a cooling chamber. , Expanding on these foundational studies, aqueous solutions containing 19.2 wt % THF were frozen to form THF hydrate while levitated in different configurations of three positions. Digital and thermal images were captured simultaneously of the single-, double-, or triple-droplet systems.…”

“…Heterogeneous nucleation occurs more favorably at higher temperatures than homogeneous nucleation, so it is the most probable mechanism and dominant mode for nucleation . However, this implies that system-specific defects on internal reactor surfaces can play a significant role in defining the rate of nucleation and hydrate growth . In turn, the effects of solid interfaces on hydrate formation measurements require additional consideration so that results can be generalized and novel hydrate systems can be scaled from the laboratory to the industrial level.…”

“…Most notably, Jeong et al formed natural gas hydrates with water droplets suspended in an acoustic levitator (i.e., levitated hydrates), where no solid interface was in contact with the liquid sample . Their investigations determined that levitated hydrates had more nucleation sites than those grown from solid surfaces, although the nucleation work was greater . Recent advances in acoustic levitation systems have allowed multiple liquid samples to be frozen simultaneously .…”