Kobayashi Takuya, Jung Gerhard, Matsuoka Yuki, Nakayama Yasuya, Molina John J, Yamamoto Ryoichi
Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan.
Laboratoire Charles Coulomb (L2C), Université de Montpellier, CNRS, 34095 Montpellier, France.
Soft Matter. 2023 Sep 27;19(37):7109-7121. doi: 10.1039/d3sm00600j.
This study presents the application of the smoothed profile (SP) method to perform direct numerical simulations for the motion of both passive and active "squirming" particles in Newtonian and viscoelastic fluids. We found that fluid elasticity has a significant impact on both the transient behavior and the steady-state velocity of the particles. Specifically, we observe that the swirling flow generated by the squirmer's surface velocity significantly enhances their swimming speed as the Weissenberg number increases, regardless of the swimming type. Furthermore, we find that pushers outperform pullers in Oldroyd-B fluids, suggesting that the speed of a squirmer depends on the swimmer type. To understand the physical origin of the phenomenon of swirling flow enhancing the swimming speed, we investigate the velocity field and polymer conformation around non-swirling and swirling neutral squirmers in viscoelastic fluids. Our investigation reveals that the velocity field around the neutral swirling squirmers exhibits pusher-like extensional flow characteristics, as well as an asymmetric polymer conformation distribution, which gives rise to this increased propulsion. This is confirmed by the investigation of the force on a fixed squirmer, which revealed that the polymer stress, particularly its diagonal components, plays a critical role in enhancing the swimming speed of swirling squirmers in viscoelastic fluids. Additionally, our results demonstrate that the maximum swimming speeds of swirling squirmers occur at an intermediate value of the fluid viscosity ratio for all swimmer types. These findings have important implications for understanding the behavior of particles and micro-organisms in complex fluids.
本研究展示了平滑轮廓（SP）方法在对牛顿流体和粘弹性流体中被动及主动“蠕动”粒子的运动进行直接数值模拟方面的应用。我们发现流体弹性对粒子的瞬态行为和稳态速度都有显著影响。具体而言，我们观察到，随着魏森贝格数增加，无论游泳类型如何，由蠕动者表面速度产生的涡旋流都会显著提高其游动速度。此外，我们发现推进型在奥尔德罗伊德 - B流体中表现优于拉动型，这表明蠕动者的速度取决于游泳者类型。为了理解涡旋流提高游动速度这一现象的物理根源，我们研究了粘弹性流体中不产生涡旋和产生涡旋的中性蠕动者周围的速度场和聚合物构象。我们的研究表明，中性涡旋蠕动者周围的速度场呈现出类似推进型的拉伸流动特征，以及不对称的聚合物构象分布，这导致了推进力的增加。对固定蠕动者上的力的研究证实了这一点，该研究表明聚合物应力，特别是其对角分量，在提高粘弹性流体中涡旋蠕动者的游动速度方面起着关键作用。此外，我们的结果表明，对于所有游泳者类型，涡旋蠕动者的最大游动速度出现在流体粘度比的中间值处。这些发现对于理解复杂流体中粒子和微生物的行为具有重要意义。
