Viscosity and mean free path of very diluted solutions of He-3 in He-4.

We have measured thc laminar friction of various diluted He-3-He-4 mixtures , of natural He-4 and of isotopically pure He-4 on an oscillating sphere be low 1 K. For He-3 concentrations x(3) ranging fron, 10(-2) to 10(-4) we fin d a reduction of the drug above 0.5 K when compared to the pure liquid and a large enhance,ment below which is almost independent of x(3). At low conc entrations 5 (.) 10(-5) > x(3) > 5 (.) 10(-7) the drag becomes proportional to x(3) which implies a transition from a hydrodynamic to a ballistic regi me. This is confirmed by deducing the mean free path of the He-3 atoms from the data. The temperature dependence. of the drag in, thr, ballistic regim e, however. is found to be proportional to T and therefore different from t he expected T-1/2 behaviour.