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Microorganisms in the human body swim in confined environments like sperm cells in the Fallopian tube or E. coli bacteria in the colon. Also pathogens use narrow channels like blood vessels or the urethra where they swim in moving fluids like a Poiseuille flow. Pushers such as sperm cells or bacteria propel themselves with flagella attached at the back of the cell body whereas pullers like the algae Chlamydomonas typically have a propelling apparatus in the front.
As a simple model microorganism we use the so-called squirmer. It has a spherical shape with a prescribed axisymmetric tangential surface velocity. We systematically investigate the swimming behavior of both the micro-swimmer in a cylindrical microchannel with an imposed Poiseuille flow. The hydrodynamics of squirmers including thermal noise is modeled using multi-particle collision dynamics. This method introduces ballistic and collision steps of effective particles in order to solve the Navier-Stokes equations. When the strength of the flow is sufficiently small, pushers swim upstream at the wall. Pullers can also swim upstream, however, in the center of the channel. Increasing the strength of the imposed flow, pushers and pullers now start to tumble. Hydrodynamic interactions with the wall become negligible and both swimmers can also perform periodic motions around the centerline of the channel while drifting downstream. These observations match well with our analytical model reminiscent to the nonlinear pendulum eqation. |
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