Behaviors of a settling microswimmer in a narrow vertical channel

An immersed boundary-lattice Boltzmann (IB-LB) method is employed to study the behaviors of a settling microswimmer in a narrow vertical channel (blockage ratio κ = 0.5). We perform the simulations with swimming Reynolds numbers (Res) between 0.1 and 3.0 and the density ratios (γ) between 1.1 and 2.3. A two-dimensional squirmer model is used to mimic a microswimmer, finding four typical locomotive modes, including the vertical mode, the attracted oscillatory mode, the oscillatory mode, and the upward mode. It is found that both the inertia and the density ratio competitively affect the locomotive modes. Increasing Res (0.1 ≤ Res<1.5) enhances the attraction of the sidewall, the vertical mode for a puller (propelled from the front) and the oscillatory mode for a pusher (propelled from the rear) develop to the attracted oscillatory mode. The pushers (−7 ≤ β ≤ −5) swim with the upward mode at Res ≥ 2.0 due to the enhanced swimming intensity generating the greater negative torque-integral when interacting hydrodynamically with the sidewall. On increasing γ, the attracted oscillatory mode of a pusher (puller) develops to the oscillatory mode (vertical mode). When initialized at the centreline of a channel along the direction of gravity, the speeds of the pushers (pullers) are enhanced (hindered) at lower Res (Res<0.59). With the increase of Res (Res>0.59), the speeds of the pushers (pullers) are hindered (enhanced). In addition, a puller swims faster at low γ whereas slower at high γ than a pusher.