Abstract
Laminar side-by-side multiphase flows are ubiquitous and indispensable in many microfluidics devices. However despite this importance they have been severely neglected and many of characteristics are assumed to be true from studies at larger scales or from intuition. Herein, we report our observations concerning rotation of the interface between vertically stratified flows with equal density, even at Peclet numbers of the order 1000 where supposedly diffusion is commonly ignored. Considering that a perfectly vertical interface is essential to the performance of related devices, the findings here suggest further study of the interface behavior is a necessary step to achieve efficient two-phase microfluidics.