When do shape changers swim upstream?

Abstract

Using a multiple-scale analysis, Walker et al. (J. Fluid Mech., vol. 944, 2022, R2) obtain the long-time behaviour of a shape-changing swimmer in a Poiseuille flow. They show that the behaviour falls into one of three categories: endless tumbling at increasing distance from the midline of the flow; preserved initial behaviour of the swimmer; or convergence to upstream rheotaxis, where the swimmer is situated at the midline of the flow. Furthermore, a single swimmer-dependent constant is identified that determines which of the three behaviours is realised.

Pre-asymptotic dispersion of active particles through a vertical pipe: the origin of hydrodynamic focusing

“…This work attempts to explore the potential transitions from one concentrative phenomenon to the other during pre-asymptotic dispersion. Given that the trajectory of an individual gyrotactic micro-organism depends on hydrodynamic and gravitational torques, the rate of change of orientation is governed generally by (Leal & Hinch 1972; Pedley & Kessler 1992; Bearon 2022) where is the gyrotactic orientation time quantifying the balance between hydrodynamic torque against viscous resistance, is the unit vector pointing vertically upwards, is the vorticity, is the shape factor, with representing spheres while denotes infinitely elongated thin rods, and is the rate-of-strain tensor. Populations of swimming micro-organisms that are initially oriented randomly and discharged uniformly in the ambient flow could react to external fields and even change their shapes periodically (Elgeti & Gompper 2015; Walker et al.…”