The Fluid Mechanics of Bacterial Swimming in Yield Stress Fluids

生物流体，如粘液，根据施加的应力可以表现为弹性固体或复杂的粘性液体。该项目的目标是了解细菌在类似生物流体的复杂流体模型中游泳的流体力学。这项全面的理论、计算和实验研究将填补理解生物流体特性与细菌运动力学之间关系的重要知识空白。游泳能力通过允许致病菌到达易感细胞的质膜并引起全身性感染，显著地促进了致病菌的毒力。因此，在该项目中获得的数据将对想要减轻或治疗细菌性疾病的研究人员有用。对于那些希望通过细菌或合成游泳者来输送药物的生物工程师来说，这也是至关重要的。此外，该项目还将通过说明工程科学的社会影响，激发K-12学生对工程的兴趣。细菌在诸如黏液之类的生物流体中的运动由于屈服应力的存在而变得复杂，屈服应力是使流体变形所必需的最小应力，流体的耦合粘性和弹性响应，以及由于细菌鞭毛的厚度与黏液链之间的间距相当而产生的非连续性。一种新的计算方法来解释这种复杂的流变性，将使用浸入边界法捕获细菌细胞，并使用细长体理论捕获鞭毛束。溶剂和聚合物将被模拟为两种相互渗透的流体，薄鞭毛只对溶剂直接施加应力。在补充实验中，纳米颗粒有机杂化生物相容性材料将被设计成具有模拟生物流体的流变学和结构。大肠杆菌悬浮液的暗场和相衬显微镜将提供统计上准确的游泳速度和细胞旋转速率分布的测量。在显微镜下跟踪单个细胞，单独的荧光染料将能够确定细胞和鞭毛束的不同旋转速率。在牛顿流体中的观察结果将用于表征驱动鞭毛的旋转马达。运动测量将与爱丁堡大学的Wilson Poon教授合作进行。实验将确定细胞游动的标准，细胞游动的速度，以及非连续流体对细胞和鞭毛施加的不同阻力，从而测试计算和理论的三个关键预测。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Biological fluids such as mucous can behave like elastic solids or complex viscous liquid depending on the applied stress. The goal of this project is to understand the fluid mechanics of bacterial swimming in model complex fluids that resemble biological fluids. This comprehensive theoretical, computational, and experimental study will fill an important knowledge gap in understanding the relationship between the properties of biological fluids and the mechanics of bacterial motility. Swimming capabilities significantly contribute to the virulence of pathogenic bacteria by allowing them to reach the plasma membrane of susceptible cells and cause systemic infections. Therefore, data acquired in the project will be useful to researchers who want to mitigate or treat bacterial diseases. It will also be crucial for bioengineers aiming to deliver drugs via bacteria or synthetic swimmers. In addition, the project will provide outreach that inspires interest in engineering among K-12 students by illustrating the societal impact of engineering science. Bacterial motility in biological fluids such as mucous is complicated by the presence of a yield stress, a minimal stress necessary to deform the fluid, the coupled viscous and elastic responses of the fluid, and the non-continuum nature arising because the bacterium’s flagella have a thickness comparable with the spacing between mucus strands. A novel computational approach accounting for this complex rheology will capture the bacterium cell using an immersed boundary method and the bundle of flagella using slender-body theory. The solvent and polymer will be modeled as two interpenetrating fluids with the thin flagella exerting their stresses directly only on the solvent. In complementary experiments, nanoparticle organic hybrid bio-compatible materials will be designed with rheology and structure that mimic the biological fluids. Dark-field and phase-contrast microscopy of suspensions of E. coli will provide statistically accurate measurements of the swimming speed and cell rotation rate distribution. In microscopy tracking individual cells, separate fluorescent dyes will enable the determination of the different rotation rates of the cell and flagella bundle. Observations in Newtonian fluids will be used to characterize the rotary motors that drive the flagella. The motility measurements will be performed in collaboration with Professor Wilson Poon at the University of Edinburgh. The experiments will determine the criterion for cells to swim, the speed of cells that do swim, and the distinct resistances the non-continuum fluid exert on the cell and flagella, thereby testing three crucial predictions of the computation and theory.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.