Disk-like Particles under Shear Flow: Understanding the physics behind yielding of colloidal disks in the nematic phase and the mechanics of red blood cell (dis)aggregation

In this thesis, the relation between the tumbling motion of disk-like particles in shear flow and the structure that the particles form is studied for two systems: Gibbsite platelets in the nematic phase and aggregates of two red blood cells.

Using time-resolved small-angle X-ray measurements, we investigate the dynamic nonlinear response to Large Amplitude Oscillatory stress and strain of a nematic dispersion of colloidal Gibbsite platelets.
By combining an X-ray beam deflected into the vertical direction with the plate-plate and the concentric cylinder Couette geometries, we track the nematic director's full 3D rotational motion.
We observe a large offset in the rheological response and an asymmetrical behavior in the microscopic structural response under controlled stress conditions.
This offset and asymmetry are connected to the yielding behavior of the platelets, they diminish for high stress amplitudes, and the microscopic response becomes more symmetric.
However, this strongly depends on the input stress frequency, hence the time necessary for the system to yield.

Confinement also strongly affects nematic Gibbsite's flow behavior due to the strong wall anchoring of colloidal disks.
We show the confinement effect by varying the gap size in plate-plate geometry and scanning the structure throughout the gap in the Couette geometry.
Analysis, using the sequence of physical processes approach, reveals indeed a gap size dependence and that the structural response is enslaved by the mechanical response.
The gap scan reveals an erratic structural response in bulk, unhindered by the wall's presence, which reduces the viscous drag.
We prove that sheared nematic Gibbsite also displays a nonlinear response in steady shear flow, both in terms of structure and local flow rate.
To this end, we introduce a new method to attain the velocity profile along the gradient in a Couette cell using X-ray Photon Correlation Spectroscopy (XPCS).

Blood is a complex fluid with strong shear-thinning behavior that depends on the ability of the red blood cells (RBCs) to form aggregates in the form of stacks, called rouleaux.
Both depletion and bridging between RBCs have long been believed to play a role in rouleaux formation, mediated by the presence of macromolecules such as fibrinogen in blood plasma.
However, despite numerous investigations, the formation and breakup of RBC aggregates have not been fully elucidated.
The most commonly used macromolecule to induce these interactions is the neutral dextran molecule.
However, experimental evidence shows that the RBCs adsorb dextran.
In order to distinguish the mechanisms behind RBC aggregation, we will employ a depletant agent with a very long-ranged interaction force, namely the filamentous fd bacteriophage.
We study the cell-to-cell interactions with the help of optical tweezers, probing the aggregation and disaggregation force between two cells as a function of the interaction time and the depletants used.
Our results show that the interaction forces driven by a pure depletant are linearly dependent on the depletant concentration versus the bell-shaped dependence displayed by dextran.
We subject pairs of aggregated RBCs to shear flow to study the breakup behavior, and we observed that indeed the doublets break up when the drag force is comparable with the aggregation force we measured previously.
However, at high shear rates, we still find doublets probably due to the complex interplay of tumbling and deformation of the cells.