The hydrodynamic transport features of a delivery system

We simulated the transport features of a delivery system, which forms by an elliptical squirmer (a type of active particle, driven by an imposed tangential velocity at its boundary) and an elliptical cargo, under walls constraints without any manual intervention. Two locomotion modes of the delivery system, including the following mode and the separating mode, are identified. The former can be detailly divided into three stability states, including the vertical stable state, the sliding stable state and the dynamic stable state. The effects of width of the box (L* = 2–3.8), self-propelling strength (β = −7–7) and aspect ratio (AR = 0.3–1) of the squirmer on the transport features [the stability, the terminal swimming velocity (U*), the power expenditure (P*), and the hydrodynamic efficient (η)] of the delivery system, are discussed. First, decreasing L* enhances the stability of all types of delivery systems, and the stability of the system driven by a pusher reducing with β while the system driven by a puller being negatively related to β at first but being independent with it when β ≥ 4. Moreover, the increasing of AR can also improve the stability of the system driven by a pusher in some extent. Second, L* (AR) is negative related to the U* of the delivery system, i.e., U* ∼ 0.3L* (∼AR−0.54), U* ∼ 0.21L* (∼AR−0.44), and U* ∼ 0.1L* (∼AR−0.3) for the systems driven by a pusher, a neutral squirmer and a puller, respectively. U* is positively with |β| for the system driven by a pusher but is negative with |β| for the system driven by a puller. Third, P* and η are negatively and positively related to L*, respectively. By contrast, they are positively and negatively related to |β|. The delivery system driven by a neutral squirmer is the most efficient transport combination, and it has the largest η at AR = 0.7.