Focusing of Active Particles in a Converging Flow

TL;DR

This paper investigates how active particles behave in a converging fluid flow within a trapezoid nozzle, revealing their trajectories, wall accumulation, and stability of upstream and downstream swimming through mathematical modeling and simulations.

Contribution

It provides a detailed analysis of active particle trajectories in converging flows, highlighting the effects of geometry and flow on particle focusing and rheotaxis, which is a novel insight.

Findings

Particles tend to accumulate near walls and exhibit upstream or downstream stability.

Convergent flow enhances particle focusing and stability of swimming directions.

The geometry and flow parameters critically influence particle trajectories and stability.

Abstract

We consider active particles swimming in a convergent fluid flow in a trapezoid nozzle with no-slip walls. We use mathematical modeling to analyze trajectories of these particles inside the nozzle. By extensive Monte Carlo simulations, we show that trajectories are strongly affected by the background fluid flow and geometry of the nozzle leading to wall accumulation and upstream motion (rheotaxis). In particular, we describe the non-trivial focusing of active rods depending on physical and geometrical parameters. It is also established that the convergent component of the background flow leads to stability of both downstream and upstream swimming at the centerline. The stability of downstream swimming enhances focusing, and the stability of upstream swimming enables rheotaxis in the bulk.