Micro-rotary ratchets driven by migratory phytoplankton with phototactic stimulus

This study proposes micro-rotary ratchets driven by a migratory phytoplankton-Volvox, exhibiting a positive phototaxis. Two types of micro-discs, i.e., ratchet- and starfish-like ratchets are fabricated using conventional photolithography. The ratchet is floated in the center of a Petri dish filled with Volvox suspension under an optical microscope with halogen lamp illumination and is covered by a mask with a small hole so that the microorganisms are concentrated around the micro-ratchets by the phototaxis. Rotations of the ratchets with the same diameter of 0.567 mm were observed through a biological microscope; a rotation speed of 0.86 rpm for the micro-ratchet and 2.01 rpm for the starfish ratchet were obtained for a Volvox density of 1000-3000/mL under an illumination intensity of 0.18 W/cm2. As the driving mechanism of the ratchet is based on the microorganisms adhesion to the ratchets surface rather than collision impacts, a gelatin coating on the ratchet was used to enhance the adhered number of Volvox. Although the drag force was increased owing to the larger ratchet diameter, a rotation speed of 0.16 rpm was observed. A particle tracking velocimetry measurement using polystyrene beads was performed to study the fluid flow around the micro-ratchet. A vortex generation by the micro-ratchets was confirmed; this effect may work as a micro-mechanical power booster for microorganisms. This drive system may open the possibility of a solar-power-driven and sustainable micro-mechanism using phytoplankton.