Complex Dynamics of driven colloidal Particles in Viscoelastic Fluids

Viscoelastic fluids are materials whose properties may vary from liquid- to solid-like
behavior. These complex fluids are part of our everyday life and are present at every
length scale. Examples include a broad range of materials e.g. blood, cosmetics, yogurt,
cement, or even the earth's mantle. Because of their complex behavior, they possess
nonlinear regimes where the fluid properties become excitation-dependent. One example
is shear thinning, where an applied stress decreases the fluid’s viscosity. As a result, their
theoretical/numerical description is quite difficult: a complete toolbox to run “numerical
experiments” has yet to be developed. As a first step, well-controlled experimental studies
are necessary to obtain a better physical understanding of such systems.

A pertinent method to study viscoelastic fluids is microrheology, a technique that uses microscopic probes to investigate properties at the microscale, and notably the role of fluctuations.

For instance, a colloidal particle (size from nanometers to microns) suspended in a Newtonian fluid exhibits Brownian motion,

which gives information on the properties of the fluid. For viscoelastic fluids, it is interesting to use active microrheology, where the probe is actively set in motion.

Experiments that measure continuous drag forces, oscillatory dynamics, or recoil effects allow to measure locally the properties of complex fluids (viscosity, relaxation time …) The objective of this PhD will be to obtain a better understanding of these materials, using an experimental approach with microrheology. It will take place within the Bechinger group at the University of Konstanz, which possess a strong experience in microrheology. The candidate will work with an already existing experimental setup that led to high quality scientific publications [1-3] . The PhD student will also participate in the development of new microrheology experiments, with the objective to look at the coupling between probe and fluid in the linear regime, and investigate the role of nonequilibrium fluctuations.

The ideal candidate would have a strong interest in Soft Matter Physics, with both theoretical and experimental skills.

Knowledge in Image Analysis and Matlab is also a plus.

Finally, he/she should be autonomous and able to run his/her own project.

[1] Oscillating Modes of Driven Colloids in Overdamped Systems
J. Berner, B. Müller, J.R. Gomez-Solano, M. Krüger, C. Bechinger
Nat. Comm. 9, 999 (2018)
[2] Probing linear and nonlinear microrheology of viscoelastic fluids
J. R. Gomez-Solano, and C. Bechinger
EPL 108, 54008 (2014)
[3] Transient dynamics of a colloidal particle driven through a viscoelastic fluid
Juan Ruben Gomez-Solano, and Clemens Bechinger
New J. Physics 17, 103032 (2015)