Bioconvection occurs as the result of the collective behavior of many micro-organisms swimming in a fluid and is realized as patterns similar to those of thermal convection, which occur when a layer of fluid is heated from below. We consider the phenomenon of pattern formation due to gyrotaxis, an orientation mechanism which results from the balance of gravitational and viscous torques acting on bottom-heavy micro-organisms. Using the continuum model of Pedley et al [“

The growth of bioconvection patterns in a uniform suspension of gyrotactic micro-organisms
,” J. Fluid Mech.195, 223 (1988)], the linear stability of a gyrotactic plume (descending line of concentrated micro-organisms) is investigated. Linear stability analysis predicts that a plume is always unstable to both the varicose and meandering modes. The growth rates of these instability modes and their dependence on parameter values are investigated. Comparisons are made with the experimental and numerical results.

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