The sedimentation behaviors of elliptical active particles in a rectangular box

Abstract

The “squirmer” model, a micro-swimming model driven by an imposed tangential velocity at the boundaries, is used to simulate the sedimentation behaviors of one and two elliptical active particles in a two-dimensional rectangular box. Six typical locomotion modes (including the steady vertically moving (SVM), steady inclined sliding (SIS), unilateral attracted oscillating (UAO), bilateral attracted oscillating (BAO), chaotic attracted oscillating (CAO), and up-down alternate tumbling (UAT)) are identified. The effects of particle aspect ratio (AR = 0.2∼1.0), self-propelling strength (β = −5∼5), swimming Reynolds number (Res = 0.1∼5.0), and density ratio of particle to fluid (γ = 1.01∼2.1) on the sedimentation behaviors of particles (the locomotion modes and the terminal Reynolds number (Ret)), are discussed. It is concluded that the locomotion modes are closely relevant to AR, γ, and Res. The collision between particles only changes their orientations but is not sensitive to the final locomotion modes. At Res≥4.0, the modes of puller type particles change from UAO to SIS or SIS to UAT. Meanwhile, it is found that (ⅰ) particle's Ret increases with AR when it is in SIS; a larger AR leads to a greater decay of Ret when it is in UAO; the decay of Ret is smooth when it is in BAO. (ⅱ) A large |β| results in a large Ret for a particle in SIS, and it is more prominent for a puller type particle than a pusher type particle. (ⅲ) In the falling period, Res speeds up a pair of pusher type particles, while it is more prominent for the pusher type particle with a lower initial position; Res also speeds up both neutral type particles with a linear relationship of Ret∼1.35Res. γ and Ret are linear positive correlated for both a pair of pusher type particles and a pair of neutral type particles.