Master Positions

Find all currently offered Master Positions down below. Click on any interesting Position to obtain detailed information about the project.

Janus microswimmers in viscoelastic media: from single particle motion to collective dynamics

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.

Phototaxis in synthetic microswimmers: Rectified motion of self-propelled particles by spatial motility variations

Phototaxis, i.e. the capability to move towards or away from light sources is an essential feature of many microorganisms like bacteria or motile cells. Unlike living systems where this is achieved by a complex internal machinery, it is not obvious how such behaviour can be imposed on synthetic microswimmers. Using colloidal Janus spheres subjected to various light landscapes, we could already demonstrate artificial phototaxis, i.e. autonomous navigation in light gradients. This leads to a strongly rectified particle current which may find use for directed particle assembly and which is also confirmed by theory and simulations. Now, we want to study in more detail this peculiar transport of particles.

Colloidal Interactions at Oil-Water Interfaces

Colloids are particles in the size range from about 10 nm to 10 µm, which are dispersed in a medium. Characteristic to them is that they are on one hand small enough to experience pronounced Brownian motion and on the other hand large enough to regard the surrounding medium as homogeneous. In daily life we meet them in form of foods, paints or cosmetic products, however at our institute we preferentially investigate well defined spherical particles made out of polystyrene or other polymers.

The properties of colloidal dispersions and emulsions result from microscopic interactions of colloidal particles among themselves and external boundaries. In a preceding Master thesis a new total internal reflection microscope (TIRM) was set up, that for the first time is capable of measuring interactions between colloidal probe particles and liquid-liquid interfaces. Using evanescent light scattering on single particles, the interactions of particles at an oil-water interface can be determined with sub 10 femtonewton (10-15 N) force resolution.

The above figure shows measured interaction potentials of attractive van der Waals forces competing with repulsive electrostatic interactions at different electrolyte concentrations. The fits according to DLVO-theory show excellent agreement with the data and the potential of the method for precise measurement of interactions.

Within this thesis in particular the electrostatic interactions with the oil water interface should be characterized further in the presence of very special salts (like NaBPh4). These salts have anions, which prefer the oil phase while the kations dissolve only in the aqueous phase. This results in interesting charge effects (Donnan potentials) at the oil-water interface.
The aim of the thesis is the first direct measurement of these potentials for colloidal particles.

Critical Casimir Forces

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.

Master position in the field of nanofriction

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).