Active substances are an important non-equilibrium system composed of self-driving units, which are ubiquitous in nature, including motor proteins at the nanoscale, to bacteria at the micron scale, to flocks of birds at the macroscopic scale. Active substances often exhibit spontaneous cluster motion behavior due to the lack of time inversion symmetry at the single-particle level. Active substance cluster behavior is an important research direction in non-equilibrium statistical physics and is one of the fields covered by the 2021 Nobel Prize in Physics.
Inspired by the active systems of life in nature, in recent years people have used simple physicochemical effects to achieve artificial active substances, such as the most common self-diffusing active colloid (micron particles drive their own movement through chemical gradients generated by catalytic reactions). Studying the cluster behavior of artificial active substances is not only of great significance for understanding the emergence of swarm intelligence, designing advanced materials and micro-nano robots, but also promising major breakthroughs in non-equilibrium physics, biomedicine and other fields.
Recently, the team led by Yang Mingcheng, researcher SM8 group of the Soft Matter Laboratory of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, together with the Ye Fangfu team and Chen Ke team of the Institute of Physics, the Dong Bin research group of Soochow University, the He Qiang research group of Harbin Institute of Technology, the Tang Jinyao research group of the University of Hong Kong and the Zheng Ning research group of Beijing Institute of Technology, have conducted in-depth exploration of the cluster behavior of artificial active substances based on the active particle system as a model, combined with theory, simulation and experiment, and achieved the following research results:
1. Light regulation of active colloidal cluster behavior
Microorganisms (such as green algae) are often able to carry out two-way phototropism in response to light intensity, that is, close to the light source in low light and away from the light source under strong light, which is essential for microorganisms to obtain food and escape harm. However, current artificial active colloids have only one-way phototropism. In order to achieve this bionic bidirectional phototropism behavior, this work combines photocatalytic-induced autodibilization and photothermal effect-induced self-heating as a propulsion mechanism for active colloids. The two self-swimming mechanisms chosen produce reverse thrusts and have different light intensity dependencies. Studies have shown that artificial active substances of this design exhibit bidirectional phototropism behavior under uniform light (Figure 1 left), while in non-uniform light fields, the competition between positive and negative phototaxis can lead to vortex cluster motion of active colloids (Figure 1). These findings represent an important step forward in the development of biomimetic photoreactive systems. Related work published in PNAS 118, e2104481118 (2021).
Light can not only cause the phototropism of active particles, but also change the effective interaction between particles. For example, a photocatalytically driven active particle can create a local chemical gradient field around it, which in turn affects the trajectory of the neighboring particle through the diffusion swimming effect, which can exhibit long-range attraction or repulsion (dependent on particle properties). Based on the effective attraction of light induction, this work realizes the dynamic regulation of the geometry and position of the active colloid cluster (Figure 1 right), providing a new way for the controlled assembly and soft material design of the active colloid. Related work published in Angw. Chem. Int. Ed. 60, 16674 (2021)。
Figure 1: Bidirectional phototropism motion of the active colloid (left), vortex cluster motion of the active colloid under (middle) non-uniform light field, and dynamic light manipulation of the active colloid cluster (right).
2. Cluster behavior of active colloids with chemical communication effects
The swarm movement of life-active substances is usually so dynamic that it responds quickly and intelligently to external stimuli (such as flocks of birds). Such dynamic cluster behavior, microscopically originated from the fact that the active monomers can communicate with each other, and then adaptively adjust their own activity. An interesting question is: Can artificial active colloids emerge with similar "intelligent" group behaviors? The ability to establish effective communication between active particles is the key to answering the above questions.
In this work, the researchers considered a mixture of two chemically driven active colloids in which the reaction products of the two active particles each acted as each other's "food" or reaction promoter, thus subtly achieving chemical communication between the two particles (Figure 2 left). When the two types of particles are closer together, they have strong activity, and conversely their activity decreases sharply (in Figure 2), so that the active particles can change their activity according to the surrounding environment. Studies have shown that such active colloids do have extremely dynamic swarm behavior similar to the morphology of bird flocks (Figure 2 right). The active system even exhibits a certain apparent "swarm intelligence": it can sense the curvature of the system boundary and spontaneously accumulate to the maximum curvature. This research provides an important foundation and basis for the development of swarm intelligence of artificial active colloidal systems. The work was published in Nature Nanotechnology 16, 288 (2021).
Figure 2: Schematic diagram of an active colloid for chemical communication (left), (medium) the distance between colloidal activity-dependent particles, and (right) bird-like behavior of an artificial active colloid.
3. Topological boundary transport of chiral active fluids
In addition to self-driven translational, active particles can also undergo self-driving rotation. The active rotor simultaneously breaks the time inversion and symmetry of the universe, and the system composed of it is called chiral active substance. Chiral active fluids have two important properties: a topologically protected collective edge flow (Figure 3 left) and a strange viscosity without dissipation. The use of traditional fluid flows to carry suspended objects in them is one of the most common modes of transporting goods. In contrast, it's not clear whether the topology edge stream has similar functionality. In order to achieve the local transport of goods in the edge flow, the goods need to be spontaneously and steadily attached to the system boundary, and then follow the edge flow directional movement. This study demonstrates that under the exhaustive force of the odd viscosity enhancement, the cargo can be stablely located at the system boundary, robustly moving along the boundary in one direction, unaffected by obstacles (Figure 3). In addition, the movement of goods can be flexibly regulated (Figure 3 right). The discovery opens up the possibility of robust transport of materials using collective marginal flows of active substances. Related work published on PRL 126, 198001 (2021).
Figure 3: Spontaneous topological edge flow of the active rotor (left), robust border transport of inactive goods (center), (right) relationship between the welt dwell time and transport speed of the cargo and rotor activity.
The above work has been funded by the National Natural Science Foundation of China and the Class B Pilot Project of the Chinese Academy of Sciences.
Article links:
Angew. Chem. Int. Ed. 60, 16674 (2021).pdf
Nature Nanotechnology 16, 288 (2021).pdf
PNAS 118, e2104481118 (2021).pdf
PRL 126, 198001 (2021).pdf
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