Abstract:
The effects caused by vapor inhomogeneity over liquid helium are considered. Both pure isotopes have surface levels, whose population increases with temperature $T$. We separated their contribution to the temperature dependence of surface tension $\sigma_3(T)$ and $\sigma_4(T)$ and compared our theoretical results with the results of Japanese experimental works [1 – 3]. For liquid He$^3$, one has $\sigma_3(T)=\sigma_3(0)-\alpha^{\infty}_3T^2$ at $0.2\,{\rm K}<T<1\,$K and $\sigma_3(T)=\sigma_3(0)-\alpha^0_3T^2\exp({-{\Delta_3}/{T}})$ at $T<0.2\,$K, with $\Delta_3\approx 0.25\,$K. For liquid He$^4$ $\sigma_4(T)=\sigma_4(0)-AT^{7/3}-\alpha^0_4T^2\exp({-\Delta_4/T})$ at $T<2\,{\rm K}$, where $A$ — is the Atkins constant and $\Delta_4\approx 4\,$K.The parameters $\alpha^0_3$, $\alpha^{\infty}_3$, and $\alpha^0_4$ depend on the fluid properties
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