An experimental study of rheology and microscopic dynamics of sheared active fluids

CBET-1702352PI：程，翔该奖项将为开发两种研究活动性流体性质的新仪器提供支持。活性流体是粒子在液体中的悬浮，可以产生自己的运动并推动自己。有人工合成的和自然产生的主动悬架。这项提议将重点放在细菌细胞的悬浮上，这种悬浮基于许多细胞的协调行为而展示出令人着迷的运动。细胞的运动导致这些液体以新的和不寻常的方式流动。明尼苏达大学将建造一台配备了能产生气流的小室的超高速共聚焦显微镜。该仪器将使研究人员能够在单个细胞的尺度上观察细胞运动，并检测流动中细胞之间发展的协调结构。Cal Poly-San Luis Obispo将建造一台薄膜流变仪。该仪器将测量主动悬架的散装特性。这些实验结果将有助于验证主动悬架的计算和理论研究。该仪器还可用于研究悬浮颗粒微观排列影响悬浮液流体动力学的其他复杂流体。调查人员将让本科生参与研究，包括来自传统上在科学和工程领域代表性较低的群体的学生。此外，他们还将在明尼苏达州博览会和加州保利的开放参观和大学前研讨会上制作新的主动暂停演示，以吸引公众。该项目将揭示微观活性颗粒动力学和活性悬浮液宏观流动行为之间的直接关联。具体地说，将对研究最广泛的活性悬浮液--大肠杆菌的悬浮液进行研究。我们将讨论两个基本问题。首先，表观粘度极低甚至为零的细菌悬浮液的超流相的动力学来源是什么？第二，外部施加的剪切流如何影响浓缩细菌悬浮液中单个活性颗粒的动力学？因此，这些实验将解决主动悬架理论预测背后的两个关键问题。这些实验将明确揭示活性流体异常流变性的微观成因，并为该领域的发展提供坚实的实验基础。该项目的成果有望帮助实践者找到更好的方法来控制生物悬浮液中的传输，防止生物膜的形成，控制悬浮液中的粘度，并改善用于生物分析和先进材料合成的微流控设备中的混合。

CBET - 1702352PI: Cheng, XiangThis award will provide support to develop two new instruments to study the properties of active fluids. An active fluid is a suspension of particles in liquid that can generate their own motion and propel themselves. There are both synthetic and naturally-occurring active suspensions. This proposal will focus on suspensions of bacterial cells, which exhibit fascinating motions based on the coordinated behavior of many cells. The cells' motions cause these fluids to flow in new and unusual ways. An ultra-fast confocal microscope equipped with a chamber that can generate flow will be built at the University of Minnesota. This instrument will allow investigators to observe cellular motion on the scale of individual cells as well as detect coordinated structures that develop among the cells in flow. A thin-film rheometer will be built at Cal Poly - San Luis Obispo. This instrument will measure bulk properties of the active suspensions. The results of these experiments will help validate computational and theoretical studies of active suspensions. The instruments will also be useful for studying other complex fluids where microscopic arrangements of suspended particles influence the fluid dynamics of the suspension. The investigators will involve undergraduates in the research, including students from traditionally underrepresented groups in science and engineering. In addition, they will create new demonstrations of active suspensions to engage the public at the Minnesota State Fair and at Cal Poly's open house and pre-collegiate symposium. This project will reveal direct correlations between microscopic active particle dynamics and macroscopic flow behaviors of active suspensions. Specifically, the most-widely studied active suspension, that of E. coli, will be investigated. Two fundamental questions will be addressed. First, what is the dynamical origin of the superfluid phase of bacterial suspensions with extremely low or even zero apparent viscosity? Second, how does an externally imposed shear flow influence the dynamics of individual active particles in concentrated bacterial suspensions? Thus, the experiments will address two key questions underlying the predictions of theories of active suspensions. The experiments will unambiguously reveal the microscopic origin of unusual rheology of active fluids and provide a solid experimental foundation for the development of the field. Results from this project are expected to help practitioners find better ways to control transport in biological suspensions, prevent formation of biofilms, control viscosity in suspensions, and improve mixing in microfluidic devices used in bioassays and advanced materials synthesis.