Self-adaptive and efficient propulsion of Ray sperms at different viscosities enabled by heterogeneous dual helixes
Significance For decades, the propulsion of sperms has been traditionally attributed to the beating or rotating of the soft tails. However, this work disclosed an unusual swimming model of Ray sperms whose propulsion is from not only the helical tail but also the unique spiral head (i.e., the head can contribute ∼31% of the force in propulsion). Such a propulsion mechanism provides Ray sperms with high adaptability to large environmental viscosities along with advantages in linearity, straightness, and efficiency. This study expands our knowledge on the microorganisms’ motion and provides inspirations for engineering design. We disclose a peculiar rotational propulsion mechanism of Ray sperms enabled by its unusual heterogeneous dual helixes with a rigid spiral head and a soft tail, named Heterogeneous Dual Helixes (HDH) model for short. Different from the conventional beating propulsion of sperm, the propulsion of Ray sperms is from both the rotational motion of the soft helical tail and the rigid spiral head. Such heterogeneous dual helical propulsion style provides the Ray sperm with high adaptability in viscous solutions along with advantages in linearity, straightness, and bidirectional motion. This HDH model is further corroborated by a miniature swimming robot actuated via a rigid spiral head and a soft tail, which demonstrates similar superiorities over conventional ones in terms of adaptability and efficiency under the same power input. Such findings expand our knowledge on microorganisms’ motion, motivate further studies on natural fertilization, and inspire engineering designs.