Blood Flow in Microcirculation: Investigating the Cell-Free Layer with Capillary Microchannels

Abstract

Blood flow in the microcirculation is governed by complex interactions between red blood cells (RBCs) and plasma, resulting in distinct rheological behaviours that differ from bulk flow properties. A key microvascular phenomenon is the cell-free layer (CFL) - a plasma-rich region near vessel walls that plays a critical role in vascular resistance, oxygen transport, and nutrient exchange. Despite its physiological significance, the primary mechanisms driving CFL formation remain debated, and accurate measurement techniques are essential for a comprehensive understanding of this phenomenon. This study aims to investigate how channel size (25 and 50 µm), hematocrit, and suspension medium influence non-Newtonian properties and CFL formation. It also seeks to evaluate the suitability of different rheological models in capturing CFL behaviour and compare measurement techniques for CFL quantification. Microfluidic experiments were conducted using blood suspensions under controlled flow conditions. Various rheological models - including the Newtonian, Power Law, and Carreau Models - were analyzed to determine their effectiveness in predicting CFL behaviour. The Double-Parameter Power (DPP) fit was also assessed for velocity profile characterization. CFL thickness was measured using both optical imaging and hydrodynamic methods, providing comparative insights into their accuracy and reliability. The results indicate that the Core-Plasma Model, developed in this work, best captures the two-phase nature of blood flow by distinguishing the RBC-rich core from the plasma layer. Optical imaging was found to be the most reliable and accurate method for CFL measurement, while hydrodynamic methods offered indirect but complementary insights. Additionally, shear rate gradients were identified as a dominant factor in CFL formation. These findings enhance the understanding of CFL dynamics in microcirculation, providing a more accurate framework for modelling blood flow under physiologically relevant conditions.