Settling dynamics of a bottom-heavy squirmer in shear-thinning fluids

Biological and synthetic micro-swimmers frequently operate in non-Newtonian environments where fluid rheology influences propulsion and transport. Many biological fluids, including mucus and blood, exhibit shear-thinning behavior, which modulates swimmer dynamics in complex ways. Although recent studies have revealed the sensitivity of heavy squirmer to such effects, their detailed settling dynamics remain underexplored. In this work, we numerically investigate the sedimentation behavior of a bottom-heavy circular squirmer in a shear-thinning fluid confined within a vertical channel. Simulations are conducted using the lattice Boltzmann method. We systematically explore the influence of key parameters: self-propulsion strength (0.1 ≤ α ≤ 0.9), swimming type (−5 ≤ β ≤ 5), density ratio between squirmer and fluid (1.1 ≤ λ ≤ 3), and power-law index (0.1 ≤ n ≤ 0.9). Our results reveal five distinct settling modes: (i) steady downward motion along the wall (SDMW), (ii) steady vertical motion along the centerline (SVMC), (iii) large-amplitude oscillatory motion (LAOM), (iv) centerline oscillatory motion (COM), and (v) down-up oscillatory motion (DOM). We find that squirmer in strongly shear-thinning fluids (n ≤ 0.7) exhibit more pronounced wall-induced oscillations than those in weakly shear-thinning fluids (n ≥ 0.8). Pusher (β 0), which generate thrust from tail to head, settles more slowly and displays larger oscillation amplitudes than puller (β 0), due to the asymmetry in flow fields interacting with local viscosity gradients. Additionally, lower density ratios λ suppress oscillations, while higher values enhance both amplitude and frequency. These findings highlight the intricate coupling between fluid rheology and swimmer dynamics, offering valuable insights for the design and control of active matter systems in complex fluidic environments.