Reorientation of the interface between two miscible solutions of equal density

•The interface between two solutions of equal density flowing in a microchannel may still rotate.•The rotation happens even at Peclet numbers of the order 103.•The diffusion coefficient imbalance between the two solutes is the main cause of the rotation.•It is possible to compensate for this rotation by creating an imbalanced density at the inlet that would try to rotate the fluid in the other direction. Horizontally stratified flows of two liquids present unique opportunities in diverse applications in microfluidics, but they also introduce new complexity into these systems; there are many microfluidic applications that fail to transition from a simplified single-phase prototype to the two-phase regime. This paper reports experimental and numerical observations that show that the quasi-interface in horizontally stratified flows might rotate even if the density is completely homogenous at the inlet and transversal diffusion is not dominant. Subsequently, based on the results it is proposes that a chain of events beginning with diffusion at the interface and resulting from simultaneous effects of diffusion, density, changes in diffusion and density due to concentration of solutes, and gravitational forces give rise to this so far unreported rotation. Simplifications that ignore any one link in the phenomena chain will break the chain of events and are hence unable to predict their combined effect. Furthermore, the flow along the channel was characterized for various values of density and velocity and one possible method to counter this phenomenon is proposed. Rotations as great as 15° were observed at Ṕeclet numbers of order 1000, which is much greater than current common practice in the field indicates. The authors believe that understanding this phenomenon and finding strategies to compensate for this effect are necessary to achieve highly efficient microfluidic devices that incorporate interfaces of miscible liquids into their design.