  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
two-dimensional water tank. Although the reciprocal flip motion of the crystals
was 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 we explore the
directionality generation mechanism and physical features of the swimming
behavior 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.
