Electrokinetic instability in co-flowing fluids with matched conductivities

Electrokinetic fluid flows with conductivity gradients have been reported to generate instability waves if the applied electric field reaches a threshold value. We demonstrate in this work that such electrokinetic instabilities can also be formed at the interface of ferrofluid and buffer solutions with matched electric conductivities. We develop a two-dimensional numerical model to potentially explain this phenomenon. It is found that the permittivity difference between the two fluids is not the primary cause of the observed electrokinetic instability. We therefore propose to use the diffusion-induced conductivity gradients that are formed at the ferrofluid/buffer interface because of the mismatched diffusivities of ferrofluid nanoparticles and buffer ions. However, due to the limitation of a two-dimensional model, our predicted threshold electric fields are much smaller than the experimental values.