Hydrodynamic modulation at the RDE

This technique was proposed by Bruckenstein and co-workers [280, 281] and is useful in that the current due to the modulation of the fluid flow is essentially free of any electrode surface-controlled contributions in most cases. Thus, it can be used as an analytical tool to increase sensitivity [282]. Step changes were originally considered but this was later extended to sinusoidal hydrodynamic modulation (SHM) in the limiting current region and then to the region of mixed convective-diffusion/kinetic control [283—287]. If the modulation frequency is o, then the modulation, which is small, can be described by 

The problem was solved numerically for the potentiostatic case and experimental verification provided [283, 284]. Analytical expressions were derived for uniform surface concentration and uniform flux if the modulation frequency is not too high [288]. Recently, the whole frequency range, including high modulation frequencies, has been reviewed with a new theoretical solution and experimental verification [289, 290], 

Besides kinetic applications, which are still to be fully realized, hydro-dynamic modulation is useful for Schmidt number and diffusion coefficient measurements not only in Newtonian fluids but also in viscoelastic polymer solutions (Ostwald fluids) [291]. 

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