Among typical active matter such as self-propelled micro-objects, the characteristic collective motion originating from the hydrodynamic interaction between constituents has been observed in both biological and artificial systems. In illustrating such motion of micrometer-size particles in a one-dimensional optically driven system with a low Reynolds number, we highlight the importance of the hydrodynamic interaction. We show the appearance of regular stationary and dynamic arrangements resembling “crystals” or “clusters” observed in the equilibrium state. A transition in the collective motion has been observed by varying the hydrodynamic interaction in a system of two particle sizes and in a spatially confined system. An optical manipulation technique and the related hydrodynamic equations are also discussed. These are useful tools for elucidating the complex collective behavior of the hydrodynamically coupled micro-objects.
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