  Micropumps are microfluidic components which are widely used in applications
such as chemical analysis, biological sensing and micro-robots. However, one
obstacle in developing micropumps is the extremely low efficiency relative to
their macro-scale counterparts. This paper presents a dynamic sealing method
for external gear pumps to reduce the volumetric losses through the clearance
between the tips of gears and the housing by using magneto-rheological (MR)
fluids. By mitigating these losses, we are able to achieve high efficiency and
high volumetric accuracy with current mechanical architectures and
manufacturing tolerances. Static and dynamic sealing using MR fluids are
investigated theoretically and experimentally. Two Mason numbers
$Mn\left(p\right)$ and $Mn\left(\Omega\right)$ which are defined in terms of
pressure gradient of the flow and velocity of the moving boundary respectively
are used to characterize and evaluate the sealing performance. A range of
magnetic field intensities is explored to determine optimal sealing
effectiveness, where effectiveness is evaluated using the ratio of volumetric
loss and friction factor. Finally, we quantify the effectiveness of this
dynamic sealing method under different working conditions for gear pumps.
