Vanishing Sticking Probabilities and Enhanced Capillary Flow of Spin-Polarized Hydrogen

We report on a series of capillary-flow experiments with spin-polarized hydrogen in which we observe a strong enhancement (> 10×) of the flow rate in comparison to ordinary Knudsen flow. This is related to a vanishing sticking probability $s$ for $T\to 0$ K as predicted by theory. With ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ surfaces we find that $s=0.33T$ for $145 \mathrm{mK}<T<\mathrm{}526\mathrm{} \mathrm{mK}$. Within experimental error $s$ did not change upon the addition of up to 50% ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at 150 mK. Results for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$/${}_{}{}^4{}_{}{}^{}\mathrm{He}$ surfaces were obtained for $73 \mathrm{mK}<T<\mathrm{}174\mathrm{} \mathrm{mK}$ and are also described by $s=0.33T$.