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NIRE Annual Report
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 1999

Formation Mechanism of Bubble Nuclear in Liquid Phase under Negative Pressure Condition

System Safety Division
Safety Engineering Department
Objectives
Bubble nuclear is a determinant of the appearance of gas phase in liquid. Although several stabilizing mechanisms of the nuclear have been suggested, the formation mechanism is not well understood. And clarifying the mechanism is very important for technical uses of bubble, such as micro-bubble utilization, measures against steam explosion and prevention of decompression sickness.
Results
The formation of gas nuclear is supposed to be too rapid and macroscopic event for direct observation. But under negative pressure condition the formation of gas nuclear was observed by the appearance of macroscopic bubble. So we used fluid inclusion as a novel system for the deliberate study of liquid under tension.
Fluid inclusions were aqueous fluids trapped in interstices in quartz and other crystals, and at ambient temperature most of inclusions had a vapor bubble in liquid. The trapped bubble was dissolved by heating and disappeared at its homogenization temperature. While cooling down the reappearance temperature of the bubble was not same with the homogenization temperature and the liquid sample was subjected to negative pressure (metastable liquid) because of the difference of thermal expansion coefficient between the host mineral and the liquid. And the negative pressure developed with cooling and finally a rapture of liquid occurred and a bubble formation restored the sample to positive pressure.
The induction times were measured to estimate the mechanism of bubble nuclear formation. Fluid inclusions were heated up until a bubble disappeared. Then it was cooled down to a slightly higher temperature than the point at which the bubble reappeared without any observable time delay, and the time was measured until the reappearance of the bubble as the induction time. Fig. 1 shows the relationship between the holding temperatures and their induction times. Each point was averaged of more than 10 measurements.
The result indicated the mechanism of generating bubble nuclear was a stochastic event. Analytical researches of molecular dynamics have been demonstrated that very small vacancies in liquid phase may be generated by the thermal agitation of molecules. These vacancies might coalesce to form a finite vapor pocket leading to nucleation, and develop to bubbles or rupture depending on its circumstance.
The study of clarifying the mechanism which induces the stochastic event will be succeeded by Raman spectroscopic observation, and so on.
Fig.1
Fig. 1. Relationship between holding temperature and induction time.

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