Hydrodynamic interaction leads to the accumulation of <i>Chlamydomonas reinhardtii</i> near a solid-liquid interface

The physical mechanism of microbial motion near solid-liquid interfaces is crucial for understanding various biological phenomena and developing ecological applications. However, limited works have been conducted on the swimming behavior of C.reinhardtii, a typical "puller" type cell, near solid surfaces, particularly with varying and conflicting experimental observations. Here, we investigate the swimming behavior of C.reinhardtii using a three-dimensional real-time tracking microscopy system both near a solid-liquid interface and in the fluid bulk region. We explore the relationships between the cell density, swimming speed and orientation with respect to the distance from the solid-liquid interface, confirming the phenomenon of C.reinhardtii accumulation near the solid-liquid interface. Based on the traditional definitions of "pusher" and "puller" cells, we propose a simplified model consisting of a pair of mutually perpendicular force dipoles for C.reinhardtii. This model is employed to analyze the complex hydrodynamic interactions between C.reinhardtii and the solid surface, providing a potential theoretical explanation for the observed accumulation phenomenon at the solid-liquid interface.