Lay summary

Colloids are nano- to micron-sized particles that are suspended in a solvent and are stabilized against aggregation by charge or by surface coatings. They do exhibit the typical phase behavior known from molecular materials with liquid, crystalline and - in many cases - glassy order. As the shape, charge, and surface properties of colloidal particles can be tailored in various ways during synthesis or by changes in their environment in the suspension medium, colloids are of great interest for many applications and also for fundamental studies. The best understood colloidal model system is that of hard spheres, colloidal particles interacting like billiard balls. Systems going beyond hard spheres in terms of shape and interaction are nowadays of largest interest in colloidal research.

This project explores the phase behavior of mixtures of magnetic and non-magnetic anisotropic particles with spherical or ellipsoidal shape. While the densest packing of spheres is known, that of ellipsoidal particles is unknown. As a consequence, the transition of ellipsoidal particles from a liquid to an ordered state is not well understood, and the orientational order of the particles is expected to be of great importance for the transition. The orientation of anisotropic magnetic colloidal particles is conveniently controlled with an applied magnetic field, such that the formation of ordered structures can be influenced also among non-magnetic particles in the vicinity of magnetic ones. Magnetic colloidal particles are of interest for applications such as magneto-rheological fluids, which change their viscosity by orders of magnitudes in an applied magnetic field.

Making use of magnetic particles, we also plan to investigate the interaction and the phase behavior of thermoresponsive microgel particles at high concentrations. Microgels are cross-linked polymer particles that respond reversibly to changes in their environment by changing their size. Thus, they exist in a swollen and a collapsed state. In the swollen state microgels are soft and easy to deform, while they behave very much like hard spheres in the de-swollen state. We will use magnetic particles covered with a microgel shell to bring particles into close contact using an applied magnetic field. This will allow to study the interaction of the microgel coronae of particles in contact. It is unknown whether the coronae do interpenetrate or deform without an overlap of the coronae.
Moreover, microgel particles containing a magnetic core will be used to induce order in a sea of nonmagnetic microgels by applying a magnetic field. The magnetic particles carry a moment that is large enough to result in the formation of chains of particles due to their magnetic dipole-dipole interaction. Such chains will be oriented in a magnetic field and their ability to serve as seeds for crystal nucleation and crystal growth will be studied. The great advantage of using microgels is the convenient control of the suspension volume fraction (concentration), which can be controlled by varying temperature.

It is the aim of the project to get insight into the phase behavior of anisotropic particles and microgel particles at high concentrations by making use of magnetic particles that are conveniently controlled by applying a magnetic field. The necessary scattering and microscopy techniques are available at Paul Scherrer Institut, and the colloidal particles have already been developed in collaboration with external partners.