We have investigated the viscosity η of liquid ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixtures at 1≤T≤100 mK and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations 0.98%≤x≤9.5%. We report on the first measurements of its pressure dependence at 0≤p≤20 bars. At 3≤T≤10 mK, the T dependence of η agrees with the Landau Fermi-liquid theory at all investigated x and p. Our results agree with calculations using quasiparticle interaction potentials V(k) derived from solubility measurements, but they disagree with predictions from pseudopotential theory. In the ballistic regime of the mixtures the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ component remains in the normal fluid state to 0.13 mK.

• Received 6 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.2761