Nucleation of Phase Separation in 3He–4He Mixtures
We report here preliminary results of experiments designed to study nucleation of phase separation in metastable thermodynamic states of liquid 3He-4He mixtures under a wide range of conditions. In addition to making some interesting qualitative observations we have been able to measure apparent lifetimes of metastable states in six solutions with mole fractions of 3He in the range 0.22 ≤x ≤0.611. In a closed container each of these solutions could be supercooled below the temperature at which it would normally phase separate (T ps) until a temperature interval less than 200µ°K wide was reached in which the apparent lifetime of the metastable state decreased more than a factor of 25 to less than 10 sec. This temperature interval occurs at much smaller supercoolings than that predicted by a classical model of homogeneous nucleation. Nevertheless, the abruptness of the decrease in lifetimes suggests the onset of some definite, temperature-dependent nucleation process. We speculate that this process is an inhomogeneous one, such as nucleation by negative ions. Our work was stimulated by two earlier experiments1,2 in which dilute solutions of liquid 3He in 4He were observed to remain in metastable states for times on the order of hours. Both of these earlier experiments were designed to study other phenomena, thus it happened that metastability was observed only below 0.1°K in solutions only slightly more concentrated than the maximum solubility of 3He in 4He at zero temperature (x ≈ 0.065).
KeywordsMetastable State Servo System Tricritical Point Free Energy Barrier Helium Mixture
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