The habitat of ciliates is widely spread and encompasses the ocean, soil, and digestive tract of mammals. Some ciliates live on surfaces; however, the effect of geometric constraints on their behavior and habitat is still largely a mystery. In this article, we discuss the behavior of swimming ciliates under various geometric constraints. To gain insight, we first calculated the lubrication forces exerted on a cell under various geometric constraints, where the cell was modeled as a squirmer. We then examined the behavior of cells near a free surface, a rigid wall, and in complex geometries. Our results show that cell behavior was strongly dependent on the geometric constraints and swimmer type; for example, some cells became entrapped, whereas others were able to escape. The mechanism underlying the behavior was well represented by the derived lubrication forces, providing a better understanding of cell behavior in natural and industrial environments. In addition, this article provides a brief overview of the field and the outlook for future research in this area.
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