Dynamics of Flexible Microfibers in Shear and Vortical Flows

At the microscale, it has been argued that phytoplankton such as diatom chains experience turbulence locally as a linear shear flow. However, the lack of vorticity gradients and vorticity transport in such interpretations can lead to oversimplification. In this work, we study the shape evolution of slender flexible fibers in different external flows. Our computational framework is a Kirchhoff rod model coupled to regularized Stokeslet segments. Previous experimental and numerical work demonstrated morphological transitions of passive elastic fibers from tumbling to S-turns to snaking in shear depending on a nondimensional elastoviscous number. We first validate our model by capturing these behaviors in shear and then transition to an analog of a Burgers vortex at zero Reynolds number.  This flow is created by the superposition of several regularized singularities of the Stokes equations. We will present model results that exhibit rich shape dynamics and excursions of flexible microfibers in these vortical flows, and will also discuss how these depend on an appropriately defined elastoviscous number.