Lay summary
The fabrication of tailored structures on the nano- and micrometer scale is one of the hot topics in soft condensed matter research using colloids,nano- or micrometer sized particles suspended in a solvent. Good control of such suspensions can be achieved, because the interactions between colloidal particles in suspension can be tailored to some extent. Liquids, crystals, gels, and glasses are commonly observed and, in many cases, phase transitions from one state to another can be induced at will by changing an easily accessible parameter such as the salt concentration of the solvent.
Suspensions of ferro- or paramagnetic colloidal particles - ferrofluids - show interesting rheological properties that can be tuned with an external magnetic field. However, ferrofluids are also of great interest due to the structural ordering of the magnetic particles that is induced by a magnetic field. Evidence of needle-like, columnar, lamellar, and pseudo-crystalline structures have been found.
Therefore, the question arises whether ferrofluids can be used for the manipulation of non-magnetic (diamagnetic) particles to induce structural order among non-magnetic particles. The exploration of the interplay between non-magnetic and magnetic colloids and their phase behavior is the goal of this project. Generally, it is found that the onset of structural ordering among non-magnetic particles is facilitated in a mixture of magnetic and non-magnetic colloids in a magnetic field. In some cases it could be shown that this effect is not due to magnetostatic interactions but due to correlations between paramagnetic and non-magnetic particles.A mayor obstacle in the exploration of the structures and phase transitions in such mixtures is the strong absorption of visible light of ferrofluids. In many cases this makes microscopy and light scattering studies impossible; both techniques are important tools in colloidal physics. Experimental techniques that are applicable to opaque samples are needed for ferrofluids; neutron- and X-ray scattering are among the most important.
The Paul Scherrer Institute is an excellent environment for this work, since these experimental tools are available.