TRANSLATIONAL OSCILLATIONS OF A MICROSPHERE IN SUPERFLUID-HELIUM

The translational motion of a microsphere (radius 100 mu m) in liquid helium is investigated. The sphere is levitating inside a superconduct ing capacitor and oscillates about its equilibrium position. The veloc ity amplitude and the resonance frequency are measured as a function o f driving force and temperature (0.35 K up to 2.2 K). By increasing th e driving force we first find a linear regime (laminar flow) which cha nges abruptly into a nonlinear one (turbulent flow). For temperatures below 0.7 K the linear drag is given by ballistic roton and phonon sca ttering whereas for temperatures above 1.1 K the hydrodynamic force on the sphere is described by Stokes' solution. In the turbulent regime, above a temperature independent threshold velocity, we find the drag force to be given by turbulence in the superfluid component plus an es sentially laminar drag by the normal component.