shear flow
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Hydrodynamic shape of microorganisms: Generalised Jeffery orbits
'Shape' of microorganisms are diverse. However, we sometimes approximate them as a sphere or a spheroid when we mathematically model the hydrodynamics of motile and non-motile cells. Such a geometrical simplification can be theoretically validated for motions in a linear background flow, since the dynamics, known as the Jeffery orbit, only contain a single geometric parameter, called the Bretherton constant. In this talk, we generalise the Jeffery equations for a chiral axisymmetric object using the low-Reynolds-number hydrokinetic symmetry and then demonstrate that the dynamics of a certain type of chiral object in a fluid flow are characterised by a new chiral parameter in addition to the Bretherton constant. We also discuss how the generalised Jeffery orbits are applied to biased locomotion of bacteria in a bulk shear flow and we will share the idea of hydrodynamic `shape' of microorganisms to simplify the description of their dynamics.
The impact of elongation on transport in shear flow
I shall present two recent piece of work investigating how shape effects the transport of active particles in shear. Firstly we will consider the sedimentation of particles in 2D laminar flow fields of increasing complexity; and how insights from this can help explain why turbulence can enhance the sedimentation of negatively buoyant diatoms [1]. Secondly, we will consider the 3D transport of elongated active particles under the action of an aligning force (e.g. gyrotactic swimmers) in some simple flow fields; and will see how shape can influence the vertical distribution, for example changing the structure of thin layers [2]. [1] Enhanced sedimentation of elongated plankton in simple flows (2018). IMA Journal of Applied Mathematics W Clifton, RN Bearon, & MA Bees. [2] Elongation enhances migration through hydrodynamic shear (in Prep), RN Bearon & WM Durham.
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