Electrokinetic instabilities in co-flowing ferrofluid and buffer solutions with matched electric conductivities

Electrokinetic instabilities have been extensively studied in microchannel fluid flows with conductivity or conductivity and permittivity gradients for various microfluidic applications. This work presents an experimental and numerical investigation of the electrokinetic co-flow of ferrofluid and buffer solutions with matched electric conductivities. We find that the ferrofluid and buffer interface becomes unstable with periodic waves if the applied direct-current electric field reaches a threshold value. We develop a two-dimensional numerical model to seek a preliminary understanding of such an electrically originated flow instability. Our model indicates that the observed phenomenon is not a consequence of the electric body force acting on the permittivity gradients between the ferrofluid and buffer solutions. It is instead attributed to the diffusion-induced conductivity gradients that are formed at the ferrofluid and buffer interface due to the mismatching diffusivities of ferrofluid nanoparticles and buffer ions.