Suspensions of swimming microorganisms play important roles in biology, medicine, and engineering. To predict and control the flow field of such suspensions, an understanding of their rheological properties is required. In this background, the suspension rheology of various types of microorganisms has been investigated intensively. Research has shown that some microorganisms, such as ciliates, deform when a strong force is exerted on their bodies. However, the effect of cell deformability on suspension rheology has not yet been clarified. In this study, we used a deformable torque swimmer, as a model ciliate, to investigate the rheological properties of a dilute suspension under shear flow. Our results show that the model swimmer tends to gradually change its orientation toward the shear plane or vorticity axis. Regardless of the swimming mode, the apparent shear viscosity shows shear-thinning properties, with the first normal stress difference being positive in sign. The second normal stress difference can be positive or negative, depending on the swimming mode, the deformability, and the shear rate. The mechanism to show such rheological properties can be understood based on the deformed shape and direction of the swimmer’s stresslet. These findings are important for understanding the suspension rheology of natural microorganisms and artificial deformable swimmers, which is essential to predict and control the flow of these suspensions.

中文翻译：

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可变形微型游泳者的稀释悬浮液的流变学

游泳微生物的悬浮液在生物学、医学和工程中发挥着重要作用。为了预测和控制这种悬浮液的流场，需要了解它们的流变特性。在此背景下，人们对各种微生物的悬浮流变学进行了深入研究。研究表明，一些微生物，如纤毛虫，当对其身体施加强大的力量时会变形。然而，细胞变形性对悬浮液流变学的影响尚未阐明。在这项研究中，我们使用可变形扭矩游泳者作为模型纤毛虫，研究剪切流下稀释悬浮液的流变特性。我们的结果表明，模型游泳者倾向于逐渐改变其朝向剪切平面或涡度轴的方向。无论游泳模式如何，表观剪切粘度显示剪切稀化特性，第一法向应力差的符号为正。第二个法向应力差可以是正的也可以是负的，这取决于游泳模式、变形能力和剪切速率。可以根据游泳者应力波的变形形状和方向来理解显示此类流变特性的机制。这些发现对于理解天然微生物和人工可变形游泳者的悬浮流变学很重要，这对于预测和控制这些悬浮液的流动至关重要。和剪切速率。可以根据游泳者应力波的变形形状和方向来理解显示此类流变特性的机制。这些发现对于理解天然微生物和人工可变形游泳者的悬浮流变学很重要，这对于预测和控制这些悬浮液的流动至关重要。和剪切速率。可以根据游泳者应力波的变形形状和方向来理解显示此类流变特性的机制。这些发现对于理解天然微生物和人工可变形游泳者的悬浮流变学很重要，这对于预测和控制这些悬浮液的流动至关重要。