Velocity measurements inside the concentration boundary layer during copper-magneto-electrolysis using a novel laser Doppler profile sensor

The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. With this new sensor, time-resolved velocity measurements within 400 μm of a vertically aligned cathode in an aqueous 0.05 M CuSO4-solution are presented. In this way, the complex interaction of Lorentz force and opposing buoyancy-driven convection was studied by measuring the resulting velocity profile inside the concentration boundary layer with a spatial resolution of 15 μm. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode. The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. Its characteristic quantities, given by the horizontal distances, δmax and δv=0δv=0, where the velocity reaches the maximum and where it is equal to zero, remain nearly unchanged, while the maximum velocity, vmaxvmax, in spite of the counteracting Lorentz force, increases faster as compared to pure natural convection, depending on the current density.

► We measure velocity profiles close to electrodes during copper-magneto-electrolysis.
► The highly unsteady initial phase is characterized by MHD and natural convection.
► The MHD flow only dominates at the early phase and is replaced by a complex mixed-flow regime due to emerging natural convection.
► The electrode-parallel velocity increases faster than for pure natural convection (j2/5-law).
► The resulting duplex boundary structure is current-independent.