In this study, we show the importance of extensional rheology, in addition to the shear rheology, in the choice of blood analog solutions intended to be used in vitro for mimicking the microcirculatory system. For this purpose, we compare the flow of a Newtonian fluid and two well-established viscoelastic blood analog polymer solutions through microfluidic channels containing both hyperbolic and abrupt contractions∕expansions. The hyperbolic shape was selected in order to impose a nearly constant strain rate at the centerline of the microchannels and achieve a quasihomogeneous and strong extensional flow often found in features of the human microcirculatory system such as stenoses. The two blood analog fluids used are aqueous solutions of a polyacrylamide (125 ppm w∕w) and of a xanthan gum (500 ppm w∕w), which were characterized rheologically in steady-shear flow using a rotational rheometer and in extension using a capillary breakup extensional rheometer (CaBER). Both blood analogs exhibit a shear-thinning behavior similar to that of whole human blood, but their relaxation times, obtained from CaBER experiments, are substantially different (by one order of magnitude). Visualizations of the flow patterns using streak photography, measurements of the velocity field using microparticle image velocimetry, and pressure-drop measurements were carried out experimentally for a wide range of flow rates. The experimental results were also compared with the numerical simulations of the flow of a Newtonian fluid and a generalized Newtonian fluid with shear-thinning behavior. Our results show that the flow patterns of the two blood analog solutions are considerably different, despite their similar shear rheology. Furthermore, we demonstrate that the elastic properties of the fluid have a major impact on the flow characteristics, with the polyacrylamide solution exhibiting a much stronger elastic character. As such, these properties must be taken into account in the choice or development of analog fluids that are adequate to replicate blood behavior at the microscale.
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机译:在这项研究中,我们显示了除了剪切流变学外,延伸流变学在选择拟用于体外模拟微循环系统的血液类似物溶液中的重要性。为此,我们比较了牛顿流体和两种公认的粘弹性血液类似物聚合物溶液通过微流体通道的流动性,该流体通道既包含双曲线收缩,也包含突然收缩。选择双曲线形状是为了在微通道的中心线处施加几乎恒定的应变率,并实现通常在人类微循环系统功能(例如狭窄)中发现的准均质且强壮的伸展流。所使用的两种血液模拟液分别是聚丙烯酰胺(125 ppm w ∕ w)和黄原胶(500 ppm w ∕ w)的水溶液,使用旋转流变仪在稳定剪切流中进行流变学表征,并使用旋转流变仪对其进行扩展。毛细管破裂延伸流变仪(CaBER)。两种血液类似物均表现出与全人类血液相似的剪切稀化行为,但从CaBER实验获得的弛豫时间却大不相同(相差一个数量级)。对于各种流速,均通过实验对条纹进行可视化显示,使用微粒图像测速仪对速度场进行测量,并对压降进行了测量。将实验结果与牛顿流体和具有剪切稀化行为的广义牛顿流体的流动的数值模拟进行了比较。我们的结果表明,尽管两种血液模拟溶液的剪切流变性相似,但它们的流动模式却有很大不同。此外,我们证明了流体的弹性特性对流动特性有重要影响,聚丙烯酰胺溶液表现出更强的弹性特性。因此,在选择或开发足以在微尺度上复制血液行为的模拟流体时,必须考虑这些特性。
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