Self-assembly is the autonomous organization of components into patterns or structures: an essential ingredient of biology and a desired route to complex organization1. At equilibrium, the structure is encoded through specific interactions2,3,4,5,6,7,8, at an unfavourable entropic cost for the system. An alternative approach, widely used by nature, uses energy input to bypass the entropy bottleneck and develop features otherwise impossible at equilibrium9. Dissipative building blocks that inject energy locally were made available by recent advances in colloidal science10,11 but have not been used to control self-assembly. Here we show the targeted formation of self-powered microgears from active particles and their autonomous synchronization into dynamical superstructures. We use a photoactive component that consumes fuel, haematite, to devise phototactic microswimmers that form self-spinning microgears following spatiotemporal light patterns. The gears are coupled via their chemical clouds by diffusiophoresis12 and constitute the elementary bricks of synchronized superstructures, which autonomously regulate their dynamics. The results are quantitatively rationalized on the basis of a stochastic description of diffusio-phoretic oscillators dynamically coupled by chemical gradients. Our findings harness non-equilibrium phoretic phenomena to program interactions and direct self-assembly with fidelity and specificity. It lays the groundwork for the autonomous construction of dynamical architectures and functional micro-machinery.
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We thank L. Bocquet and P. Chaikin for enlightening discussions. This material is based on work supported by the National Science Foundation under Grant No. DMR-1554724. J.P. thanks the Sloan Foundation for support through grant FG-2017-9392. S.S. acknowledges support from the NSF CAREER award DMR-1653465.
The authors declare no competing interests.
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Supplementary Methods, Supplementary Figures 1–6, Supplementary References 1–12
Superstructure made of seven rotors. Four rotors rotate clockwise and three anticlockwise. Their relative arrangement is random. The structure is confined by an isotropic light pattern and the dynamics is sustained under illumination. When the light is turned off, the system returns to equilibrium and the order is destroyed as the dissipative interactions vanish. Real-time video, bright field transmission illumination.
Superstructure made of seven co-rotating rotors and collective rotation. We confine seven co-rotating rotors, which form a hexagonal pattern and show a collective rotation along the direction of the common spin (clockwise). The edge-current travels at Ω ∼ 0.1 rad s–1, and persists over the entire experiment (duration ∼10 min). The dynamics is a result of the anisotropic, short-range chemical interactions between the rotors. To enhance the collective rotation, the movie is momentarily accelerated 3× at time t = 10 s, before returning to real time at t = 36 s. The video is obtained by reflected illumination.
Assembly of seven rotors with alternating spins, constituted of three rotors rotating clockwise and four anticlockwise. The structure remains static without collective rotation. Alternatively, pairs of counter-rotating neighbouring rotors synchronize. The video is accelerated 3× at time t = 10 s and goes back to real time at t = 33 s. The movie is obtained by reflected illumination.
Sequential formation of rotors and superstructures. Phototactic swimmers are dispersed in the solution and exhibit persistent random walk under homogeneous illumination. We create multiple rotors by superimposing a focused laser spot to a uniform activation light. Laser ON, the swimmers gather, collide, and swiftly form rotors composed of seven particles. The laser is switched OFF to be displaced and switched ON again to form a new rotor. The sequence is repeated to obtain three rotors with high yield and control. Finally the rotors are confined by an isotropic light pattern with a flat central region to form a superstructure. As the light is switched off, the system returns to equilibrium and the rotors break. Video is accelerated 2×, bright field transmission illumination.
Rotational dynamics of a pair of co-rotating rotors. Co-rotating rotors revolve at Ω ∼ 0.1 rad s–1, in the direction of the spins of the rotors. Real-time video. Bright field transmission illumination.
Synchronized dynamics of a pair of counter-rotating rotors. The movie is initially in real time and switches to stroboscopic, at a frequency matching the rotation rate of a rotor, at t = 10 s. The phase locking and cogwheel-like behaviour is apparent. Bright field transmission illumination.
Higher level superstructure formed of two contra-rotating sets of three co-rotating rotors. We form two contra-rotating sets of three co-rotating rotors and combine them using an anisotropic light pattern. This leads to the formation of a colloidal machine with the synchronous motion of the two sets as gears. The video is accelerated 10× at time t = 13 s and goes back to real time at t = 49 s. The video is obtained by reflected illumination
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Aubret, A., Youssef, M., Sacanna, S. et al. Targeted assembly and synchronization of self-spinning microgears. Nature Phys 14, 1114–1118 (2018). https://doi.org/10.1038/s41567-018-0227-4
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