Ferrofluid rotary seals are mechanical contact-free magnetic liquid seals that are characterised by their simple structure, low friction and ability to hermetically seal. Although ferrofluid rotary seals for sealing vacuum and gases are part of a well established industry, the sealing of liquids has not been implemented yet. Literature learns that degradation of the ferrofluid seal over time when it dynamically contacts a liquid results into premature seal failure. This thesis presents a new type of ferrofluid rotary seal in which a ferrofluid replenishment system is implemented that renews the ferrofluid in the sealing ring while sealing capacity is maintained. By replacing the degraded ferrofluid in the seal at a sufficient rate, service life of the ferrofluid rotary seal can theoretically be unlimited. An analytical model and FEM analysis are used to design the ferrofluid sealing device and to predict its sealing capacity. An experimental test setup has been built on which the sealing capacity and service life of the device has been tested for different sealing conditions. It is demonstrated that the ferrofluid replenishment system successfully extends and controls the service life of the ferrofluid rotary seal that dynamically seals pressurised water.

Magnetorheological fluids, (MR fluids) have been around since the 1940's. After their discovery many applications followed, such as the MR clutch and MR damper. In these applications the fluids are subjected to ultra high shear rates although no knowledge is available on what happens to the rheological properties under these conditions. In general, the characteristics determined on lower shear rates are extrapolated and used to design new devices. Magnetorheological fluids have potential in the high tech and high precision applications and their properties need to be known in particular at shear rates around 106 [1/s]. Commercially available magnetorheometers are not able to measure these fluids at ultra high shear rates and are limited to 105 [1/s]. Therefore a new magnetorheometer is required to measure ultra high shear rates. In this thesis the physical limitations of current measuring principles are analysed and a concept is designed for ultra high shear rate rheometry in combination with a magnetic field. A prototype is fabricated and the techniques used are described. The prototype is tested and compared to a state of the art commercial rheometer. The test results of the prototype rheometer for MR fluids show its capability to measure fluids to a range of 10^4 [1/s] to 1.16x10^6 [1/s] and the capability to measure the magnetorheological effect of MR fluids.

Ferrofluid bearings provide a cheap and passive alternative to contactless bearings such as magnetic bearings and fluid bearings. The absence of stick-slip and the low viscous friction make them interesting for high precision applications. Despite the theoretical models available in literature, the number of design rules for ferrofluid bearings is fairly limited, making it difficult to compare ferrofluid bearings to other bearing concepts.

This thesis presents new design rules for ferrofluid bearings based on an experimentally validated (FEM) model. The model calculates the equilibrium position of the bearing based on the air mass inside the pocket and the strength of the ferrofluid seal. Finally, a sensitivity analysis was performed in order to see how various errors affect the overall performance of ferrofluid pocket bearings.

Seals are used throughout the technology spectrum to prevent products form leaving their container or preventing foreign products to enter. With the sealing of rotary parts like shafts the seal needs to not only provide sealing capabilities, but also simultaneously allow for the desired movement without too much friction or wear. This is often accomplished using rubber rings in combination with lubricating oils.

The focus of this thesis is on the design and physics of ferrofluid seals, which doesn't cause wear to the shaft and is inherently leak free. A major problem with ferrofluid seals is that when the medium to seal is (sea)water, the fluid tends to degrade over time and leads to failure of the seal. The liquid reacts with the particles or carrier, causing a deterioration of the ferrofluid's properties. Research shows a seal life time can be obtained of up to roughly 300 days, as opposed to rubber lip seals holding out for several years.

A method described in this thesis is used to provide a longer life time of the seal. Analytical methods are used to determine the critical pressure and are compared with the setup to judge the performance of the setup.