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活性物质是典型的非平衡态系统，其组成单元能够利用自身存储的能量或者周围环境的能量实现自驱动。在活性系统中，物体间的受力情况直接影响其结构和动力学行为，因此深入了解物体间的有效作用力是理解活性物质一切复杂现象的基础。本文通过光镊显微镜实验分别研究了活性大肠杆菌溶液中惰性球形聚苯乙烯胶体粒子间和板状粒子间的有效作用力，发现球形粒子间有效作用力的性质一直是短程排斥力，而板状粒子间的有效作用力则为长程吸引力，这说明惰性粒子间的有效作用力受粒子形状的影响。惰性粒子间的有效作用力主要来源于两部分的贡献：细菌-惰性粒子间的直接碰撞，以及细菌运动产生的流场。我们在实验上通过对比粒子之间、粒子外侧细菌的浓度和取向有序性，发现球形粒子间的有效排斥力主要来源于细菌-粒子的直接碰撞，而板状粒子间的长程吸引力则主要源于细菌流场的贡献。本文通过光镊显微镜实验证明了惰性粒子间的有效作用力与惰性粒子的几何构型有关，为调控活性物质中的动态自组装提供了实验支撑。In active matter, the effective force between passive objects is crucial for their structure and dynamics, which is fundamental to understand the complex behaviors within active systems. Unlike equilibrium states, factors such as the surface configuration, size, and confinement strength significantly influence the effective forces between passive particles. Previous studies have shown that the shape of passive particles affects the aggregation of active particles, leading to different forces experienced by passive particles with different shapes. However, recently, Ning et al. discovered that a long-range attractive force between passive platelike particles, caused by the bacterial flow field instead of the direct bacterium-plate collisions in active bacterial suspensions. This raises an intriguing question: how does hydrodynamics differently affect the forces on passive particles of different shapes?
In this work, we investigated the effective forces exerted on passive spherical and platelike particles immersed in bacterial suspensions by optical-tweezers experiments. The effective force between passive particles can be calculated by the formula, Feff=k<△d>/2, where <△d> represent the difference of the distance between passive particles in the bacterial bath compared to the solution without bacteria, k is the effective stiffness of optical traps.Feff＞0 indicates a repulsive force between passive particles, and Feff＜0 represents an effective attractive force between passive particles. Our results demonstrate that the passive spherical particles experience short-range repulsion, while platelike particles exhibit long-range attraction. This highlights the substantial impact of particle shape on their effective forces.
The forces on passive particles are primarily attributed to two factors: direct bacterium-particle collisions and the bacterial flow field. Analysis of the bacterial concentration and orientation distribution around passive particles reveals that for spherical particles, the bacterial concentration is higher between particles than outside the particles, yet there is little difference in the orientation order of bacteria between inside and outside the particles. This suggests that the effective repulsion between spherical particles is mainly due to the direct bacterial collisions. Conversely, for platelike particles, the long-range attraction is primarily influenced by the bacterial flow field rather than direct collisions, which is evidenced by the higher bacterial density and orientation order inside the two plates compared to outside that. This study provides strong evidence that the effective force between passive particles is shape-dependent in active bath, and offers new insights into controlling active-directed assembly.-
Keywords:
- active matter /
- optical tweezers /
- the effective interaction /
- particle shape
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