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活性物质中的集体行为已被广泛研究. 然而, 活性个体间的吸引力对集体运动模式的影响还没有相关研究. 本研究发现, 在Quincke活性胶体体系, 通过增强电液流(EHD)效应诱导的长程吸引力, 速度对齐效应得到增强, 并在极性流体内部诱发密度相分离, 形成极性团簇, 破坏极性有序相的空间均匀性. 这是已有的实验观测中没有的. 而在排斥主导的活性体系, 极性流体空间均匀分布. 我们的研究结果表明活性个体间的吸引力会显著改变活性体系的微观和宏观动力学行为.
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关键词:
- Quincke活性胶体 /
- 吸引力 /
- 动态团簇 /
- 集体运动
Active matter can form various collective motions. In dry and repulsive systems, a uniform polar fluid emerges in the presence of an aligning mechanism. Theoretical studies have shown that in active systems with attractive interactions, particles can achieve spontaneous velocity alignment and form clusters through the synergistic effect of self-propulsion and attraction. However, so far, the influence of attractions on collective behavior has not been well addressed experimentally. In this work, an electric-field driven Quincke system, where an electrohydrodynamic (EHD) long-range attraction is present, is used to investigate the influence of attraction on the collective behavior. It is found that the long-range attraction can significantly increase the interaction time during collision, thereby enhancing velocity alignment. The aligned particles can form dynamic polar clusters. Moreover, in the presence of a long-range attraction, a uniform polar fluid is unstable: density fluctuation leads to denser polar clusters which share the same direction of collective motion with the polar fluid. Our findings show that the attraction between active individuals can significantly change the microscopic and macroscopic dynamics of active systems and provide insights into understanding chemotactic attraction phenomena in biological systems.-
Keywords:
- Quincke rollers /
- attractive interactions /
- dynamic clusters /
- collective motion
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图 1 两粒子(P1, P2)的碰撞过程($ E = 1.20{E_{\text{c}}} $) (a) 实验装置及原理图; (b) 湿AOT导电溶液中粒子碰撞过程; (c)干AOT导电溶液中粒子碰撞过程. (b), (c)顶部: 粒子之间的距离随时间的变化. 插图: 对齐过程中的轨迹与速度变化. 中间: 两个粒子速度方向之间的角度随时间的变化. 底部: 两粒子速度变化
Fig. 1. Collision dynamics between two particles ($ E = 1.20{E_{\text{c}}} $): (a) Schematic of the experimental setup and Quincke rolling; (b) particle collision process in wet AOT conductive solution; (c) particle collision process in dry AOT conductive solution. (b), (c) Top: Distance between the particles as a function of time. Inset: Speed and trajectories in an aligning process. Middle: The angle between the two orientations as a function of time. Bottom: Speed evolution in pair aligning.
图 2 (a) 排斥力占主导时的无序气体 ($ E = 1.20{E_{\text{c}}} $); (b)—(d) 吸引力主导下形成的团簇及其消散过程($ E = 1.20{E_{\text{c}}} $); (e)—(h)对应于图(a)—(d)的速度方向着色图, 左下角圆盘为方向色标. 图中标尺为$100{\text{ μm}}$
Fig. 2. (a) Repulsion-dominated disordered gas state ($ E = 1.20{E_{\text{c}}} $); (b)–(d) attraction-driven cluster formation and subsequent dissipation($ E = 1.20{E_{\text{c}}} $); (e)–(h) velocity direction color maps corresponding to panels (a)–(d). The direction color key is shown in the lower-left disk. Scale bar: $100{\text{ μm}}$.
图 3 吸引Quincke体系的亚临界行为 (a) $ E = 0.9{E_{\text{c}}} $时的实验原图; (b)对应于图(a)的速度彩图, 图中白色粒子为静止不动的粒子, 右侧为速度大小色标. 图中标尺为$100{\text{ μm}}$
Fig. 3. Subcritical behavior of the attraction-driven Quincke system: (a) Raw experimental image at $ E = 0.9{E_{\text{c}}} $; (b) velocity color map corresponding to Figure (a), white particles denote stationary colloids, and the velocity color scale is shown on the right. Scale bar: $100{\text{ μm}}$.
图 4 集体运动 (a) 排斥力占主导时的极性流体($ E = $$ 1.30{E_{\text{c}}} $); (b) EHD吸引力占主导时的极性团簇 ($ E = 1.30{E_{\text{c}}} $); (c), (d)对应于图(a)和图(b)的速度方向着色图, 左下角圆盘为方向色标. 图中标尺为$100{\text{ μm}}$
Fig. 4. Collective motion: (a) Repulsion-dominated polar fluid ($ E = 1.30{E_{\text{c}}} $); (b) EHD attraction-driven polar cluster ($ E = 1.30{E_{\text{c}}} $); (c), (d) velocity direction color maps corresponding to Figure (a) and Figure (b). The directional color key is shown in the lower-left disk. Scale bar: $100{\text{ μm}}$.
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