Permanent magnet systems for microfluidic applications

The content of this thesis ranges from magnetic material properties, to magnet
designs, shimming methods to optimize their magnetic field homogeneity and a
variety of applications of permanent magnet systems.
The research in chapter 2 aimed to compare a cuboid and a Pseudo-Halbach
magnet configuration, in terms of their field strength and field homogeneity. For
our optimized boundary conditions, we advise using the cuboid configuration
for scientific use, where it is possible to preselect the permanent magnets and
the external stray field is not a big issue. If preselecting the magnets is not an
option, we recommend the Pseudo-Halbach configuration, which has a more robust
homogeneity regarding variations in the magnetization and angle between the used
magnets. Also the lower stray field makes this configuration easier to handle and
therefore more favorable especially for industrial applications.
In chapter 3 and 4 we present three different ways to improve the field homogeneity
of the Pseudo-Halbach. Using mechanical shimming, in our case two
movable rings, brings two advantages.
First is a higher field, second a way to shim the field by sliding the rings closer
together or further apart, depending of the aimed region of interest. This design is,
caused by the amount of magnets and additional cage material to keep the magnets
in place, more expensive than the earlier mentioned configurations but due to the
easy handling an enjoyable magnet to work with.
We further improved the field with two planar shimming chips which uses two
striplines and one conductive ring each, to generate a field which gets superimposed
with the magnet field, resulting in a much better homogeneity.
The third method is a pilot study which aims for a better field-homogeneity
by using a small amount of ferrofluid in a microfluidic chip. We have shown the
possible usage of this method by alternating the volumes and the saturation
magnetization of the fluid. With including more advanced fabrication techniques
for the microfluidic chips, this method could be used in future for minimized
magnet designs where the fabrication accuracy of permanent magnets is reached.
In chapter 5 we present a microfluidic Coriolis sensor, whose gain could be
improved with either placing a magnet right on top of the chip, or placing a
Halbach-like configuration around the channel.
Chapter 6 shows a new technique to sort magnetic microparticles by the Magnus
force. Using two parallel magnets to spin the particles results in a drift, which
deflection side depends on the spinning direction. With this technique, the particles
could be sorted depending on their size and/or weight.