Application of population balance-based thixotropic model to human blood.

• Application of a multiscale, population balance‐based thixotropy model to blood. • More physical description of the coin‐stack like rouleaux aggregate structures. • Meaningful microscopic structural information obtained by fitting bulk rheology. Modeling blood rheology remains challenging in part because of its multiphase, aggregating colloidal nature that gives rise to complex viscoplastic and time-dependent (thixotropic) behavior. Here, we demonstrate that a multiscale approach incorporating a direct coupling of coarse-graining particle-level modeling to the macroscopic phenomenological modeling can provide new insights and a promising methodology. Specifically, a general population balance-based, multiscale, thixotropic modeling approach, first proposed by Mwasame et al., AIChE J. 63 (2017) 517–531, is applied to account for the rouleaux-induced thixotropy in human blood in shear flow. Population balances offer a compelling alternative to previously proposed structure-based heuristic kinetics models of aggregating colloidal suspensions as they use a statistical approach to describe the aggregate size distribution with well-defined processes for either shear-induced or Brownian aggregation and breakup under shear flow. When applied to human blood, the population balance approach offers a first attempt to model the size evolution of predominantly coin-stack like rouleaux structures of the red blood cells that are the primary source behind the observed yield stress and thixotropy of blood at low shear rates. This microscopic information, suitably coarse-grained, is then introduced into a semi-phenomenological macroscopic model that expresses the total stress in terms of an elastic and viscous contribution. Shear thinning introduced due to the red blood cell deformation at high shear rates is accounted for by following Horner et al., J. Rheol. 62 (2018) 577–591. An advantage of this modeling approach is that the parameters have specific physical meaning that allows for independent estimates and/or evaluations through appropriately designed independent experiments. Conversely, parameters with specific microscopic interpretations, such as the fractal dimension of the aggregates, d f , are obtained fits of macroscopic shear experiments. Fitting and predictions use steady shear, and unidirectional large amplitude oscillatory shear (UD-LAOS) experiments on whole blood samples of two healthy donors, as reported in Horner et al. We obtain values for d f in the range of 1.5 ± 0.2 which is consistent with the rod-like shape of rouleaux structures reported in the literature. Furthermore, the shear predictions compare favorably against the experiments. While this approach is not as accurate as the fits of prior structure kinetics modeling of Horner et al., these promising results provide a pathway for model improvement by including independently verified physical properties of blood. This work demonstrates a new particle-level approach for describing and predicting the non-Newtonian, thixotropic rheology of human blood. [ABSTRACT FROM AUTHOR]