Self‐Propulsion of a Light‐Powered Microscopic Crystalline Flapper in Water

A key goal in developing molecular microrobots that mimic real‐world animal dynamic behavior is to understand better the self‐continuous progressive motion resulting from collective molecular transformation. This study reports, for the first time, the experimental realization of directional swimming of a microcrystal that exhibits self‐continuous reciprocating motion in a 2D water tank. Although the reciprocal flip motion of the crystals is like that of a fish wagging its tail fin, many of the crystals swam in the opposite direction to which a fish would swim. Here the directionality generation mechanism and physical features of the swimming behavior is explored by constructing a mathematical model for the crystalline flapper. The results show that a tiny crystal with a less‐deformable part in its flip fin exhibits a pull‐type stroke swimming, while a crystal with a fin that uniformly deforms exhibits push‐type kicking motion. Self‐propulsion of a submillimeter‐sized crystal of molecular motor in a water pool is realized under continuous‐wave visible light irradiation. The crystal drives itself by self‐oscillatory reciprocal flapping motion. Whether the swimmer excels at the stroking or kicking style is governed by slight differences in the outline of the crystals.