(460c) Viscoelasticity, Thixotropy, and Wall Effects in Human Blood Rheology

Blood is a complex suspension primarily composed of concentrated red blood cells suspended in an aqueous plasma phase. At rest and low shear rates, the red blood cells will form linear, coin stack aggregates called rouleaux. These structures enable blood to demonstrate a variety of interesting rheological signatures including a nonzero yield stress, viscoelasticity, and thixotropy – a time dependent viscosity. However, measuring these effects at low shear rates can often be difficult due to the presence of a slip layer near the walls of the measurement device. This phenomenon is known as syneresis and is particularly notable when rouleaux are present due to the large size of the structures as well as the significant difference between the plasma and bulk viscosities. At high shear stresses, the red blood cells can deform, giving rise to another distinct viscoelastic component. Understanding the rheology of blood is critical for both improving blood flow simulations throughout the circulatory system as well as diagnosing, treating, and preventing a variety of cardiovascular diseases as numerous diseases have been linked to enhanced rouleaux formation.

Recently, we have collected a number of data samples following a carefully developed handling protocol [1] for both steady and transient bulk rheology on human blood. Using these data, we have developed a model to capture the viscoelasticity and thixotropy associated with human blood rheology [2]. This model was unique in that it was the first model for human blood rheology that was able to capture two distinct elastic contributions – one associated with the rouleaux contributions at low shear rates and one associated with the deformation of isolated red blood cells at high shear rates. Additionally, this model was shown to be capable of predicting the stress response of blood over a range of steady and oscillatory flow conditions and outperformed comparable literature models for blood rheology [2].

In this work, we first present additional rheological data on stress growth and relaxation experiments that allow us to better identify the elastic rouleaux contribution to the bulk stress. Using the previous and the new data, we have further improved our blood flow model. Moreover, we also introduce a model for the depletion layer at the walls of the measurement device to account for the effects of syneresis. The depletion layer is directly linked to the nondimensional structure parameter which governs the rouleaux contribution to the bulk rheology and enables an indirect tracking of the rouleaux size for different flow states. In addition to modeling this depletion layer formation, we have also been able to improve our previous formulation of the elasticity associated with the rouleaux. The improved formulation is a nonlinear, viscoelastic model which accounts for the stretching of rouleaux under shear, the structural breakup at high shear stresses, and the relaxation of the rouleaux when the flow is reduced.

The enhanced model is compared to experimental results for steady, oscillatory, and step shear change rheological tests and significantly improves upon the previously published model predictions. Constitutive modeling of thixotropy, viscoelasticity, and slip layer formation is not only important for blood rheology but can be used for a wide range of similar concentrated colloidal suspensions. Additionally, by accurately modeling the rheology of blood, we can improve blood flow simulations throughout the circulatory system, and by linking the model parameters to the blood physiology, we can improve our understanding of how physiology and general health are linked to the flow behavior of blood.

This work is supported by the National Science Foundation, award number CBET 1510837.

[1] Horner J. S., A. N. Beris, D. S. Woulfe, and N. J. Wagner, Clin. Hemorheol. Microcirc., (In press).

[2] Horner J. S., M. J. Armstrong, N. J. Wagner, and A. N. Beris, J. Rheol., 62(2), (2018).