Study on the effect of geometric shape on microswimmer upstream motion

Abstract

The upstream motility of three microswimmer shapes (circular squirmer, squirmer rod, and elliptical squirmer) at the center of a Poiseuille flow is numerically investigated using the lattice Boltzmann method. Based on the stability and upstream ability, the swimming velocities and four motion states (stable motion, progressively unstable motion, unstable motion, and upstream failure) are summarized. The results show that the circular squirmer and squirmer rod are more stable than the elliptical squirmer; however, the elliptical squirmer has the greatest advantage in velocity and can swim up to twice as fast as the circular squirmer under the same conditions. The swimming type is also the key to influencing the motion state, which is reflected differently in the distinct microswimmer shapes. The increase in the Reynolds number (Re) and self-propelled strength (α) aggravates the motion instability; however, for elongated microswimmers, the aspect ratio (ε) plays a role in velocity rather than the motion state. Moreover, the upstream velocity of the pusher is always better than that of the puller, especially when Re increases. Notably, all microswimmers can maintain stable swimming when the preset velocity is twice the maximum velocity of the flow field. These findings can provide guidelines for the selection of design parameters and the appearance of microswimmers that resist complex incoming flows.