Settling characteristics of squirmers in a vertical channel of power-law fluids

Abstract This study uses the lattice Boltzmann method to investigate the sedimentation dynamics and interactions of two circular active particles (squirmers) in a two-dimensional vertical channel under the influence of gravity. Four distinct locomotion modes are observed for individual particle: oscillations along the centreline, large-amplitude oscillations, downward motion along the wall, and steady vertical descent along the centreline. The effects of swimming type (characterized by the squirming parameter β , −5 ≤ β ≤ 5), self-propulsion strength ( α , defined as the ratio of swimming speed to settling velocity, −0.1 ≤ α ≤ 0.9), and fluid rheology (described by the flow behavior index n , 0.5 ≤ n ≤ 1.5) on sedimentation behavior are systematically analyzed. The results indicate that squirmers exhibit larger oscillations in Newtonian ( n = 1) and shear-thinning ( n < 1) fluids compared to shear-thickening ( n > 1) fluids. Pushers ( β < 0), which generate flow away from their heads, exhibit larger amplitude oscillations, whereas pullers ( β > 0), which direct flow toward their heads, display smaller and more stable oscillations. The interaction dynamics between two squirmers differ significantly from those of a single squirmer, with asymmetrical pressure distributions playing a crucial role. Pushers settle faster than pullers due to the negative flow generated at the tail. Squirmers aligned with the direction of gravity tend to attract each other and may form contacts through regions of low pressure. Additionally, oscillatory behaviour is amplified at higher density ratios ( γ ), while lower ratios result in reduced fluctuations. These findings offer crucial insights into the coupled rheological properties of fluids and the dynamics of active particles, with significant implications for the design and control of active matter systems.