Viscosity and Mean Free Path of Very Diluted Solutions of 3He in 4He

We have measured the laminar friction of various diluted 3He-4He mixtures, of natural 4He and of isotopically pure 4He on an oscillating sphere below 1 K. For 3He concentrations x3 ranging from 10−2 to 10−4 we find a reduction of the drag above 0.5 K when compared to the pure liquid and a large enhancement below, which is almost independent of x3. At low concentrations 5·10−5>x3≥5·10−7 the drag becomes proportional to x3 which implies a transition from a hydrodynamic to a ballistic regime. This is confirmed by deducing the mean free path of the 3He atoms from the data. The temperature dependence of the drag in the ballistic regime, however, is found to be proportional to T and therefore different from the expected T1/2 behaviour.