title: Deformation of a micro-torque swimmer.
abstract: The membrane tension of some kinds of ciliates has been suggested to regulate upward and downward swimming velocities under gravity. Despite its biological importance, deformation and membrane tension of a ciliate have not been clarified fully. In this study, we numerically investigated the deformation of a ciliate swimming freely in a fluid otherwise at rest. The cell body was modelled as a capsule with a hyperelastic membrane enclosing a Newtonian fluid. Thrust forces due to the ciliary beat were modelled as torques distributed above the cell body. The effects of membrane elasticity, the aspect ratio of the cell's reference shape, and the density difference between the cell and the surrounding fluid were investigated. The results showed that the cell deformed like a heart shape, when the capillary number was sufficiently large. Under the influence of gravity, the membrane tension at the anterior end decreased in the upward swimming while it increased in the downward swimming. Moreover, gravity-induced deformation caused the cells to move gravitationally downwards or upwards, which resulted in a positive or negative geotaxis-like behaviour with a physical origin. These results are important in understanding the physiology of a ciliate's biological responses to mechanical stimuli.
authors.affiliation: Department of Bioengineering and Robotics , Tohoku University , 6-6-01, Aoba, Sendai 980-8579, Japan.
authors.forename: Takuji
authors.initials: T
authors.lastname: Ishikawa
authors.affiliation: Department of Bioengineering and Robotics , Tohoku University , 6-6-01, Aoba, Sendai 980-8579, Japan.
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Tanaka
authors.affiliation: Department of Bioengineering and Robotics , Tohoku University , 6-6-01, Aoba, Sendai 980-8579, Japan.
authors.forename: Yohsuke
authors.initials: Y
authors.lastname: Imai
authors.affiliation: Department of Bioengineering and Robotics , Tohoku University , 6-6-01, Aoba, Sendai 980-8579, Japan.
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Omori
authors.affiliation: Department of Bioengineering and Robotics , Tohoku University , 6-6-01, Aoba, Sendai 980-8579, Japan.
authors.forename: Daiki
authors.initials: D
authors.lastname: Matsunaga
keywords: Stokes flow; ciliate; fluid–solid interaction; locomotion