Master Positions

The motion of many natural microswimmers, such as bacteria, algae and spermatozoa, commonly takes place in viscoelastic media due to the presence of suspended polymers, macromolecules and colloids. 
The understanding of their swimming mechanisms has triggered a lot of experimental and theoretical work in recent years, in particular to the development of artificial self-propelled particles. Despite their biological and application-related relevance, most experiments with microswimmers have been performed in Newtonian liquids. In our experiments, using colloidal Janus particles in a polymer mixture in combination with laser illumination we have achieved the first experimental relization of autonomous synthetic microswimmers moving in viscoelastic fluids. In this project we want to investigate in detail new non-equilibrium phenomena that emerge from the coupling between such self-propelled particles and the slow relaxation of the surrounding fluid.

We are currently seeking for a Masterstudent for creation of complex colloidal structures using critical Casimir forces. Such forces are induced by concentration fluctuations in a binary mixture near its critical point. One of the most striking features of such interactions is the tunability by small temperature variations which offers novel strategies in colloidal assembly.

Friction is an important phenomenon which occurs whenever two surfaces are in relative motion. Despite its general importance, however, the fundamental mechanism of friction is not well understood. This applies particularly to small length scales where experiments are difficult to perform. In our group we address this issue by sliding colloidal monolayers across periodic substrates, the latter created by interference patterns which exert optical forces on the colloids. In previous experiments we could already demonstrate the occurrence of topological solitions during sliding which provide an efficient mass transport between surfaces in relative motion [1]. Now, we want to study the effect of additional substrate excitations on the frictional properties and to explore under what conditions superlubric sliding will occur.

[1] T. Bohlein, J. Mikhael, and C. Bechinger, Observation of kinks and antikinks in colloidal monolayers driven across ordered surfaces, Nature Materials 11, 126 (2012).