  Ferrofluids kept in place by permanent magnet quadrupoles can act as liquid
walls to surround a second non-magnetic inside, resulting in a liquid fluidic
channel with diameter size ranging from mm down to less than 10 micrometer.
Micro particle tracking velocimetry (micro PTV) experiments and modeling show
that near ideal plug flow is possible in such liquid-in-liquid channels due to
the reduced friction at the walls. The measured fluids velocity profiles agree
with the predictions of a hydrodynamic model of cylindrical symmetry with a
minimal set of hypotheses. By introducing symmetry breaking elements in the
system, we show how unique velocity and flow properties can be obtained. Our
liquid-in-liquid confinement opens new possibilities for < 10 micrometer-sized
microfluidics with low pressures and low shear, with flow characteristics not
attainable in comparable solid-wall devices.
