Finite-Size Effects and Shear Viscosity in SuperfluidHe3-B

The effective viscosity of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ has been measured by means of a torsional pendulum with a characteristic dimension of 135 μm. A comparison between the experimental results and theory—including the effects of (normal) fluid slip, Knudsen flow, and Andreev scattering of quasiparticles from spatial variations of the order parameter—reveals that while the experimental results at high pressure are bounded by the theoretical extremes considered, the low-pressure data are not; in fact, the disparity is seen to increase at lower pressures and lower temperatures.