Trapping micro-swimmers over a cavity in an inertial micro-channel

Based on the lattice Boltzmann method and the squirmer self-propulsion model, this paper focuses on the motion characteristics of three types of swimmers (neutral swimmer, puller, pusher) over a cavity in an inertial micro-channel. The effects of fluid inertia (Re_f) and channel-cavity structural parameters (L_d, L_w) on the capture and separation of micro-swimmers are analysed. The results indicate that there are five motion modes of swimmer under low Reynolds number. As fluid inertia increases, three new motion modes emerge. Neutral swimmer is captured at low Re_f, while the puller and pusher are captured at high Re_f. Under a suitable cavity aspect ratio, the puller also exhibits an additional new trapping mode known as the circle loop trapping. Moreover, the pusher with large activity will behave differently at various Reynolds numbers. Ultimately, through data and theoretical analysis, theoretical formulas are developed to reflect the effect of cavity size and Reynolds number on the capture and separation performance through the phase diagram. By integrating the examination of diverse trapping behaviours of microwimmers within complex microchannel structures, this study significantly enhances our understanding of the complex motion of microwimmers in intricate flow environments, and facilitates the manipulation and control of manmade microwimmers.