  Mixing remains an important problem for development of successful
microfluidic and lab-on-a-chip devices, where simple and predictable systems
are particularly interesting. One is magnetic micro-convection, an instability
happening on the interface of miscible magnetic and non-magnetic fluids in a
Hele-Shaw cell under applied field. Previous work proved that Brinkman model
quantitatively explains the experiments. However, a gravity caused convective
motion complicated the tests. Here we first improve the experimental system to
exclude the gravitational convective motion. Afterwards, we observe and
quantify how gravity and laminar flow play an important role in stabilizing the
perturbations that create the instability. Accordingly, we improve our
theoretical model and perform linear analysis. Two dimensionless quantities
explain the experimental observations of change in critical field needed for
instability and characteristic size of the emerging pattern. Finally, we
discuss the conditions at which gravity plays an important role in microfluidic
systems.
