Shuttling of charge by a metallic sphere in viscous oil

We measure the current transported by a microscopic metallic sphere shuttling charge through a fluid between two electrodes. Metallic spheres with diameters of 90 - 200 μ m were formed by emulsification of the alloy wood's metal above its melting point in electrically insulating oil. Droplets of oil containing a single sphere in suspension were placed between two parallel electrodes. After a voltage was applied, the sphere conducted current through the oil by moving back and forth between the electrodes. The measured current was proportional to the voltage raised to a power that ranged between 3 for relatively high-viscosity silicone oil and slightly above 2 for hexadecane. The magnitude of the current and the frequency of the shuttling are well described across a broad range of parameters by a model that includes dissipation in the fluid at low Reynolds number. Discontinuous jumps in the conductance arise from the finite inertia of the particle, which leads to recoil from the electrodes at Stokes number of order ten.