弱剪切微流管道中细菌的三维跟踪、集群运动及动力学研究

Micro-swimmer of bacteria perform the self-propelled motion and transport in solution or the biological fluids, which underpins the motility and activity from the micro to macro scale for the living systems and our life. Understanding the mechanisms behind this self-propelled phenomenon, collective motion toward controlling and exploiting the microbe is an essential aspect. Recently most of the theoretical, numerical and experimental attempts are attracted on this active motion under static fluidic surroundings, which will be remarkably different when this motion is subjected to the external flow, particularly the flow with the extremely low shear rate and a comparable mean velocity to the bacteria motility. Under this weak flow, we can ask various open questions on how bacteria use the rheotaxis strategy accounting for the boundary surfaces, what are their ‘personality’ including sensing and reacting to deform the flagella and body, then to adapt themselves from the run-tumble strategies, which are great intriguing and crucial to be explored. Therefore, by introducing the Poiseuille flow in the microfluidic channels with the careful control of the low shear rate, and by developing the 3D tracking thanks to the defocusing techniques, we try to understand the more precise details of the individual trajectory for deriving their ‘personality’ behaviour of the bacteria suffering from the flow by the physical quantities such as the probability density function of the mean square displacement and velocity, spatial–temporal curvature of the swimming trajectories, and the persistent length, induced run-tumble time variation, and memory lost of the bacteria, then the onsite for the collective motion coupled bacterial population and flow shear rate. Thereafter using the information of trajectories, critical flow rate under concentrated bacterial suspension, we relate the distribution, aggregation and collective motion induced by the bacterial self-propelled motion to the formation and instability of the biofilm, aiming to find the physical way to control this colony formation for the possible solution toward the biofouling and health problems.

细菌在流体或生命体内的自驱动运动与物质输运，是支撑许多微观与宏观生命活动的核心，理解这种自驱动作用机制、集群运动及操控是一个极为重要的问题。当前自驱动研究侧重于静态流场中的运动，然而引入外加流场，尤其当流场与细菌游动速度可比、极低剪切时，边界处细菌驱流性导致聚集及顽固性疾病生物膜，细菌的“个性”自我响应流场并调节运动取向，以及鞭毛几何形状与细菌椭球体的不对称对其平动与转动扩散的影响等物理问题将变得新奇有趣且至关重要。基于此，本项目拟引入微弱流场、极低剪切率的泊肃叶流，构建基于“散焦”技术的拉格朗日参考系下高分辨高速跟踪技术，展示自驱动粒子运动“个性”化的精细运动轨迹，研究个体细菌在弱流场下反常自驱动机制，及集群运动对细菌数密度、临界剪切的依赖，建立集群运动、聚集与生物膜的物理关联，寻找一种控制集群的形成，从而在顽固性生物膜及由此产生的细菌抗药性与微生物污染上探索出流场调控的物理方法。

背景：细菌通过手性鞭毛与环境流体作用获得定向位移，然而细菌总是不可避免地生存于有边界的流场扰动环境中，与梯度流场相互作用、如何自我响应流场等物理问题是本项目的关键科学问题。. 研究内容：本项目系统研究了左手螺旋鞭毛细菌在泊肃叶剪切流中游动特性，发展了拉格朗日运动参考系下高分辨高速示踪方法，展示细菌运动“个性”化的精细运动轨迹。从单个细菌游动拓展到单体强相互耦合作用下集群运动，并系统研究了微流管道、梯度空间、多孔凝胶等受限条件下细菌运动统计规律。. 重要结果与数据：.（1）构建了拉格朗日法细菌显微示踪模型系统平台，测定构建了随体坐标系下细菌如何游动的精细细节，定义了全新的手性无量纲数，从而诠释了细菌运动手性驱流普适规律。（Science Advances, 6, eabb2012，2020）.（2）通过剪切流场、粒子扩散调控、蒸发诱导的毛细流相互耦合，构建了具有不同边界条件的微流受限空间系统，并确立了细菌游动、细胞的定向迁移运动规律。（J. Phys. Chem. C, 123, 7408-7415, 2019; New J. Phys., 22, 053005, 2020; Angew. Chem. Int. Ed. 59: 5611-5615, 2020）.（3）建立了杆状细菌在DLVO边界势能作用下不存在普适的状态方程；利用细菌湍流诱导的涡旋，显著提升扩散混合；明确了液滴接触线附近细菌湍流能谱在长波下具有普适标度律，发现短波下具有显著的空间梯度限制效应。（Acta Physica Sinica, 68:170501, 2019; MicroMachine, 11:195, 2020; Phys. Rev. Fluids, 2022 under review）. 研究意义与学术贡献：.首先，我们确立手性作用项的精确表达式，首次定义了无量纲“手性”数，获得了驱流运动的普适标度律，重构了手性粒子在流场中运动的全景图。这为一大类手性细长粒子在流场中精确运动轨迹估算、微尺度下自驱动粒子导航提供了理论依据与实验方案。.其次，精确确立细菌3D空间运动与分布，以及界面上细菌动态囚禁与逃离，为广泛存在的细菌感染医疗应用提供了重要依据与抑菌、滤菌提供了可行物理手段。.最后，细菌湍流的物理规律与调控，为微流管道中层流混合小尺度流场反应、增强超扩散提供了新方案。