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This run features an aqueous solution that is approximately 15.5 wt.% Na2SO4. Initially, the pressure was about 0.4 MPa. 05/09/19 17:15:52: The run starts around T = 294 K. There were still some mirabilite crystals near the bottom of the screen left over from the previous run. Upon initial cooling, those crystals grow from the bottom of the window. (Gravity points down.) The temperature decreases, but the volume slowly increases as the crystal grow. (Though the crystals are more dense than the surrounding fluid, the remaining fluid is significantly less dense, leading to an overall slight increase in volume.) The light was initially dim, but was gradually increased to make those crystals visible. 05/09/19 20:51:20: The remaining fluid suddenly freezes. The screen goes dark. Since the Ice Ih expands upon freezing, the volume increases dramatically, and the pressure goes up as well. Next, the system will be warmed up to eventually reach the eutectic temperature. Since the connecting tubing is also frozen, neither the pressure nor volume transducer accurately reflects conditions in the growth cell. The pressure inside is much higher. 05/09/20 11:19:20: Some crystals start to melt at T = 252.44, consistent with what would be expected if the internal pressure were quite high -- around 190 MPa. 05/09/20: 14:43:20: As the solid melts and the material in the connecting tubing softens, the pressure goes down somewhat, and there is even some re-solidification. As the system is gradually warmed, this sort of behavior happens again. 05/09/21 17:25:20: Enough material has melted so that the transducer is finally free to move, and the pressure and volume readings are again reflective of conditions inside the pressure cell. As the system is gradually warmed, the mirabilite crystals at the bottom start to dissolve. 05/09/21 21:04:20: Upon cooling very slightly to T = 271.85, crystals start growing rapidly again. 05/09/22 07:53:20: Upon warming, melting starts again around 271.99. The tight volume/temperature loop shows that this is a reversible eutectic melting transition. 05/09/22 08:17:38: A small air bubble is visible in the system. At higher pressures, no such bubbles were observed. 05/09/22 16:08:20: The volume had been dropping steadily as Ice Ih melted, but now the ice is all gone and the only remaining solids are the mirabilite crystals at the bottom of the cell. These gradually dissolve as the temperature is raised. As the crystals dissolve, the volume goes down. 05/09/25 11:50:20: Eventually, all the solid is dissolved; the remaining liquid undergoes modest thermal expansion as the temperature continues to rise up through the end of the run. The lenses in the optical tube eventually get covered with condensation and the image grows fainter.
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