Convection caused by three-phase boundary reactions

When a ferrocene (Fc)-included nitrobenzene droplet was set on an electrode immersed in an aqueous solution, the electrochemical oxidation of Fc at the interface of nitrobenzene∣water∣electrode gave rise to the convection, which flowed from the top of the hemispherical nitrobenzene droplet on the electrode to the three-phase boundary both in the nitrobenzene and aqueous phases. The flow velocity was determined by tracing the dispersed carbon powders with a video microscope. The convection was ascribed to two asymmetries of the flow: the dragging force on nitrobenzene by the diffusing species (Fc) toward the three-phase boundary without any counter diffusion of the product (Fc+), and the difference in the frictional force near the oil∣electrode boundary and the oil∣water boundary. The convective diffusion equation was combined with Navier–Stokes’ equation associated with the diffusion force of Fc in the oil phase. The combined equations were solved semi-quantitatively on the assumptions of the one-dimensional stream model. The solution explained the potential dependence of the flow velocity.