Search

We present numerical analysis of the lateral movement of spherical capsule in the steady and pulsatile channel flow of a Newtonian fluid, for a wide range of oscillatory frequency. Each capsule membrane satisfying strain-hardening characteristic is simulated for different Reynolds numbers Re and capillary numbers Ca. Our numerical results showed that capsules with high Ca exhibit axial focusing at finite Re similarly to the inertialess case. We observe that the speed of the axial focusing can be substantially accelerated by making the driving pressure gradient oscillating in time. We also confirm the existence of an optimal frequency which maximizes the speed of axial focusing, that remains the same found in the absence of inertia. For relatively low Ca, on the other hand, the capsule exhibits off-centre focusing, resulting in various equilibrium radial positions depending on Re. Our numerical results further clarifies the existence of a specific Re for which the effect of the flow pulsation to the equilibrium radial position is maximum. The roles of channel size and viscosity ratio on the lateral movements of the capsule are also addressed.

The deformability of human red blood cells (RBCs), which comprise almost 99% of the cells in whole blood, is largely related not only to pathophysiological blood flow but also to the levels of intracellular compounds. Therefore, statistical estimates of the deformability of individual RBCs are of paramount importance in the clinical diagnosis of blood diseases. Although the micro-scale hydrodynamic interactions of individual RBCs lead to non-Newtonian blood rheology, there is no established method to estimate individual RBC deformability from the rheological data of RBC suspensions, and the possibility of this estimation has not been proven. To address this issue, we conducted an integrated analysis of a model of the rheology of RBC suspensions, coupled with macro-rheological data of human RBCs suspended in plasma. Assuming a non-linear curve of the relative viscosity of the suspensions as a function of the cell volume fraction, the statistical average of the membrane shear elasticity was estimated for individual intact RBCs or hardened RBCs. Both estimated values reproduced well the experimentally observed shear-thinning non-Newtonian behavior in these suspensions. We hereby conclude that our complementary approach makes it possible to estimate the statistical average of individual RBC deformability from macro-rheological data obtained with usual rheometric tests.

We present numerical analysis of the lateral movement of a deformable spherical capsule in a pulsatile channel flow, with a Newtonian fluid in almost inertialess condition and at a small confinement ratio $a_0/R = 0.4$, where $R$ and $a$ are the channel and capsule radius. We find that the speed of the axial migration of the capsule can be accelerated by the flow pulsation at a specific frequency. The migration speed increases with the oscillatory amplitude, while the most effective frequency remains basically unchanged and independent of the amplitude. Our numerical results form a fundamental basis for further studies on cellular flow mechanics, since pulsatile flows are physiologically relevant in human circulation, potentially affecting the dynamics of deformable particles and red blood cells (RBCs), and can also be potentially exploited in cell focusing techniques.

Biomembranes consisting of two opposing phospholipid monolayers, which comprise the so-called lipid bilayer, are largely responsible for the dual solid-fluid behavior of individual cells and viruses. Quantifying the mechanical characteristics of biomembrane, including the dynamics of their in-plane fluidity, can provide insight not only into active or passive cell behaviors but also into vesicle design for drug delivery systems. Despite numerous studies on the mechanics of biomembranes, their dynamical viscoelastic properties have not yet been fully described. We thus quantify their viscoelasticity based on a two-dimensional (2D) fluid membrane model, and investigate this viscoelasticity under small amplitude oscillatory loadings in micron-scale membrane area. We use hydrodynamic equations of bilayer membranes, obtained by Onsager's variational principle, wherein the fluid membrane is assumed to be an almost planar bilayer membrane. Simulations are performed for a wide range of oscillatory frequencies $f$ and membrane tensions. Our numerical results show that as frequencies increase, membrane characteristics shift from an elastic-dominant to viscous-dominant state. Furthermore, such state transitions obtained with a 1-$\mu$m-wide loading profile appear with frequencies between $O(f) = 10^1-10^2$ Hz, and almost independently of surface tensions. We discuss the formation mechanism of the viscous- or elastic-dominant transition based on relaxation rates that correspond to the eigenvalues of the dynamical matrix in the governing equations.

We present a numerical analysis of the lateral movement and equilibrium radial positions of red blood cells (RBCs) with major diameter of 8 $\mu$m under a Newtonian fluid in a circular channel with 50-$\mu$m diameter. Each RBC, modelled as a biconcave capsule whose membrane satisfies strain-hardening characteristics, is simulated for different Reynolds numbers $Re$ and capillary numbers $Ca$, the latter of which indicate the ratio of the fluid viscous force to the membrane elastic force. The effects of initial orientation angles and positions on the equilibrium radial position of an RBC centroid are also investigated. The numerical results show that depending on their initial orientations, RBCs have bistable flow modes, so-called rolling and tumbling motions. Most RBCs have a rolling motion. These stable modes are accompanied by different equilibrium radial positions, where tumbling RBCs are further away from the channel axis than rolling ones. The inertial migration of RBCs is achieved by alternating orientation angles, which are primarily affected by the initial orientation angles. Then the RBCs assume the aforementioned bistable modes during the migration, followed by further migration to the equilibrium radial position at much longer time periods. The power (or energy dissipation) associated with membrane deformations is introduced to quantify the state of membrane loads. The energy expenditures rely on stable flow modes, the equilibrium radial position of RBC centroids, and the viscosity ratio between the internal and external fluids.

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) for different volume fractions in a wall-bounded, effectively inertialess, small amplitude oscillatory shear (SAOS) flow for a wide range of applied frequencies. The RBCs are modeled as biconcave capsules, whose membrane is an isotropic and hyperelastic material following the Skalak constitutive law. The frequency-dependent viscoelasticity in the bulk suspension is quantified by the complex viscosity, defined by the amplitude of the particle shear stress and the phase difference between the stress and shear. SAOS flow basically impedes the deformation of individual RBCs as well as the magnitude of fluid-membrane interactions, resulting in a lower specific viscosity and first and second normal stress differences than in steady shear flow. Although it is known that the RBC deformation alone is sufficient to give rise to shear-thinning, our results show that the complex viscosity weakly depends on the frequency-modulated deformations or orientations of individual RBCs, but rather depends on combinations of the frequency-dependent amplitude and phase difference. The effect of the viscosity ratio between the cytoplasm and plasma and of the capillary number are also assessed.

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) in a wall-bounded shear flow. The flow is assumed as almost inertialess. The suspension of RBCs, modeled as biconcave capsules whose membrane follows the Skalak constitutive law, is simulated for a wide range of viscosity ratios between the cytoplasm and plasma: $\lambda$ = 0.1-10, for volume fractions up to $\phi$ = 0.41 and for different capillary numbers ($Ca$). Our numerical results show that an RBC at low $Ca$ tends to orient to the shear plane and exhibits the so-called rolling motion, a stable mode with higher intrinsic viscosity than the so-called tumbling motion. As $Ca$ increases, the mode shifts from the rolling to the swinging motion. Hydrodynamic interactions (higher volume fraction) also allows RBCs to exhibit both tumbling or swinging motions resulting in a drop of the intrinsic viscosity for dilute and semi-dilute suspensions. Because of this mode change, conventional ways of modeling the relative viscosity as a polynomial function of $\phi$ cannot be simply applied in suspensions of RBCs at low volume fractions. The relative viscosity for high volume fractions, however, can be well described as a function of an effective volume fraction, defined by the volume of spheres of radius equal to the semi-middle axis of the deformed RBC. We find that the relative viscosity successfully collapses on a single non-linear curve independently of $\lambda$ except for the case with $Ca \geq$ 0.4, where the fit works only in the case of low/moderate volume fraction, and fails in the case of a fully dense suspension.