Flowing complex fluids
When flow is applied to complex fluids then it can cause structural changes and induce phase transitions. But the fluid can also alter the flow, causing flow instabilities. This complex interplay between microstructure and flow is studied for various systems of anisotropic colloids: rods, stiff supra-polymers, platelets and red blood cells.
Red Blood Cells under Flow
Rouleaux are the cylindrical structures that spontaneously form in our blood when there is no flow and which cause our blood to have a high yield stress. In order to understand the flow of our blood it is therefore of fundamental importance to characterize the build up and break up of these rouleaux. We investigate using a fundamental physical-chemical approach to tune interactions this effect as well as our home-build zero-velocity plane shear cell to create ideal flow conditions and 3D microfluidics to create physiological flow conditions [Korculanin2021].
A confocal video of red blood cells in shear flow, showing the off-pinching of two membrane-connected cells.
Rods under Shear
Dispersions of colloidal rods can show very complex behavior under shear flow, depending on the concentration regime. While nematic as well as the dilute suspensions have received a lot of attention, the simple shear thinning response in the semidilute case is surprisingly neither well studied nor understood, even though this is the most very relevant range in biology and industrial application. We gain insight in this behavior be combining biochemistry, producing a library of filamentous bacteriophages with different length and stiffness, 3D rheo-SANS from two different directions, providing 3D particle distributions under shear flow, and elaborated theory [Lang2019].
Viscosity and orientation vs. a scaled shear rate for varying concentrations of rods, comparing old and new theory.
The Molecular Origin Underlying Shear Thinning of Polymers
To resolve the microscopic origin of the shear thinning mechanism in polymeric systems, we perform confocal microscopy under flow of F-actin of several tens of microns long. This allows to probe the time-resolved configuration of the polymer chains in full 3D. We find, for example, that shear thinning is accompanied by a layering of the chains in planes perpendicular to the vorticity direction [Kirchenbuechler2014].
Time resolved confocal projections of fluorescence-labelled F-actin filaments from two different directions. The length of the shown F-actin is 20 micron.
Shearing Nematic Platelets
Most soft matter materials cannot be classified as fluids or solids because they possess a dual character: they can have a response that is solid-like or fluid-like, depending on the mechanical deformation. This dual, visco-elastic, character can also be found in an important class of soft matter systems, namely liquid crystals. Liquid crystals can be formed by small ‘colloidal’ platelets, such as clay particles, which all have similar orientation, so that they are in a nematic phase despite of their Brownian motion, which makes them colloidal. We subjected these dispersions of platelets to an oscillating shear field, squeezing the dispersions between two moving plates. By combining this geometry with time-resolved X-ray scattering, we uncover the structural origin of the transformation between solid and liquid-like behaviour and in a cascade of complex flow responses. These new insights might change the view on the origin of visco-elastic behaviour in complex fluids [Korculanin2021,Chen2021].