Field-induced assembly in magnetic systems - towards tunable magnetic devices

Self-assembly is an umbrella term for a fascinating range of processes by which initial components build into a complex structure via a chemical or physical change. From complex inorganic macromolecules to the tertiary structure of polypeptides, self-assembled systems represent an extremely wide variety of systems. However, controlling self-assembly remains a formidable challenge, not least because there are usually many components present which lead to a range of different interactions and competing driving forces. To exploit self-assembled structures for the development of useful materials and tunable devices, it is a prerequisite to have deep understanding of the fundamental structural and dynamical processes underlying the self-assembly.

One particular class of material that displays a rich variety of self-assembled patterns is termed magneto-rheological (MR) fluids. MR fluids are formed by magnetic colloidal nanoparticles dispersed in a non-magnetic solvent (or non-magnetic colloidal nanoparticles in a magnetic fluid). Upon the application of an external magnetic field typically chain-like and fibrous structures appear, which sensitively affect the viscosity of the system. This effect is exploited in a wide range of applications ranging from MR seismic shock dampers to innovative prosthetic limbs. Furthermore, with their tunability of interaction, MR fluids are excellent model systems for fundamental network forming systems.

Here, we propose to investigate the field-induced assembly of fibrous network materials in MR fluids using experiments, computer simulations and stochastic fibrous network theory. Because the typical colloidal length and time scales are of the order of micrometers and seconds respectively, the structure and dynamics can be analysed at the particle level using optical microscopy, which allows for detailed comparisons between experiments, simulations and theory. Furthermore, we aim to exploit the effect of the addition of impurities, confinement and flow to control the network formation, which will allow us to explore the development of tunable magnetic devices.

The proposed research will shed detailed light on the formation of fibrous network structures in magneto-rheological (MR) fluids. The results of this research are relevant to a wide range of industrial and technological beneficiaries, because MR fluids, fibrous materials and colloidal systems in general are encountered in many applications ranging from daily life examples as foods and inkjet printing to technological applications as coatings, security printing, shock-absorbers, and optical materials. Hence, many companies are direct beneficiaries of this research. Just a few examples include Toppan (security printing), Kodak and Philips (optical materials), ICI/Akzo Nobel (coatings) and the paper industry. The improvement of the products and applications of these companies will have an indirect beneficial effect on the users of these products in the UK and abroad.

The beneficiaries will profit in various ways from this research. Understanding the behaviour of colloidal systems is relevant in the development of a host of products like paints, coatings and inks. More specifically, magneto-rheological fluids have special flow properties which make them attractive for applications as tunable shock absorbers and lubricants. In addition, the ability to manipulate the flow of complex MR fluids using external magnetic fields is of vital importance for successfully mixing and structuring products at the micron scale in microfluidic devices. Because this research will significantly contribute to the control of self-assembly in magneto-rheological fluids, our results will be of direct technological benefit to companies that develop highly tunable materials and microfluidic devices.

The role of impurities in magneto-rheological systems is also of direct interest to the development of new materials. For example, heterogeneities in ceramic materials are usually caused by impurities and this sensitively changes the electrical and mechanical properties of functional ceramics. The fundamental insight into the effect of impurities on fibrous network formation is therefore beneficial for the improvement of colloidal processing of ceramic materials. The effect of confinement is also becoming increasingly important in many technological applications due to the continuing miniaturization of devices. Understanding the impact of confinement on fibrous network formation is hugely relevant for the successful development of advanced optical materials and nanofilters, of which the performance is extremely sensitive to the pore-size distribution of the material.

In addition to the developed technological expertise, we train highly skilled researchers in this specific area. Companies will benefit from the availability of these specialists, which will have a positive impact on their customers and on the UK economy. To establish the engagement of companies, the applicants have a number of contacts with industry. Collaborations with Beiersdorf and Unilever are ongoing and MatOx has expressed its interest in the use of magnetic colloidal systems for enhanced oil recovery purposes. Also, active involvement in SoftComp, a European Network of Excellence, gives the applicants direct access to important multinationals like BASF and Rhodia. Dissemination to a wider public will be achieved using the active public engagement programs of the Universities of Oxford and Manchester, which for instance includes a science blog. The `Pathways to Impact' described further details on what will be done to ensure that the beneficiaries have the opportunity to benefit from this research.