Ordering and arrangement of deformed red blood cells in flow through microcapillaries

The shapes and alignment of elastic vesicles similar to red blood cells (RBCs) in cylindrical capillary flow are investigated by mesoscopic hydrodynamic simulations. We study the collective flow behavior of many RBCs, where the capillary diameter is comparable to the diameter of the RBCs. Two essential control parameters are the RBC volume fraction (the tube hematocrit, H-T), and the suspension flow velocity. Depending on H-T, flow velocity and capillary radius, the RBC suspension exhibits a disordered phase and two distinct ordered phases, consisting of a single file of parachute-shaped cells and a zigzag arrangement of slipper-shaped cells, respectively. We argue that thermal fluctuations, included in the simulation method, coupled to hydrodynamic flows are important contributors to the RBC morphology. We examine the changes to the phase structures when the capillary diameter and the material properties (bending rigidity kappa and stretching modulus mu) of the model RBCs are varied, constructing phase diagrams for each case. We focus on capillary diameters, which range from about 1.0 to about 1.4 times the RBC long diameter. For the smallest capillary diameter, the single-file arrangement dominates; for the largest diameter, the ordered zigzag arrangement begins to loose its stability and alternates with an asymmetric structure with two lanes of differently oriented cells. In simulations with long capillaries, the coexistence of different phases can be observed

Deformation and clustering of red blood cells in microcapillary flows

The shape changes and clustering of red blood cells (RBCs) under flow in cylindrical microcapillaries are studied using a triangulated surface model for the membrane and a particle-based mesoscopic simulation technique for the embedding fluid. As the flow velocity increases, the RBCs make a transition from a discocyte shape at low velocities to a parachute shape at high velocities; close to the critical flow velocity, the RBC can also be found in a transient slipper shape. The transition and critical flow velocity are examined for various capillary diameters and RBC volume fractions (hematocrit H-T). At high flow velocities and low hematocrits, the parachute-shaped RBCs can be found in clusters which are hydrodynamically stabilized. Here, the formation of a fluid vortex between neighboring cells, called bolus, develops which keeps the cells at a preferred distance. Decreasing the flow velocity towards the critical velocity, we observe an increasing frequency of drastic RBC shape fluctuations to slipper-shaped RBCs that can result in cluster breakup. These clusters resemble those seen in experiments using optical microscopy

Mechanical properties of cellulose nanofibrils determined through atomistic molecular dynamics simulations

Abstract We have carried out atomistic molecular dynamics simulations to study the mechanical properties of cellulose nanofibrils in water and ethanol. The studied elementary fibrils consisted of regions having 34 or 36 cellulose chains whose cross-sectional diameter across the fibril was roughly 3.4 nm. The models used in simulations included both crystalline and non-crystalline regions, where the latter were designed to describe the essentials parts of amorphous cellulose nanofibrils. We examined different numbers of connecting chains between the crystallites, and found out that the elastic constants, inelastic deformations, and strength of the fibril depend on this number. For example, the elastic modulus for the whole fibril can be estimated to increase by 4 GPa for each additional connecting chain.</jats:p