(474c) Margination and Segregation of Self-Propelled Particles in Blood Flow

Self-propelled particles like swimming bacteria and microbots have been envisioned for various applications in the human body including drug delivery and minimally invasive surgery. The efficacy of these particles in their respective applications will depend critically on their flow behavior in blood vessels. A property of particular interest will the distribution of the particles in the wall normal direction, i.e., whether the particles flow close to the walls (marginate) or close to the vessel center (antimarginate). In this work, we numerically examine the flow behavior of self-propelled particles in a suspension of deformable capsules, the latter being a model for red-blood-cells. This binary suspension is subjected to simple shear flow in a slit geometry, i.e., between two parallel walls. The effect of a variety of parameters is characterized   including the particle shape, aspect ratio, size, swimming speed and the mode of propulsion, i.e., whether the particles are pushers or pullers. Our studies indicate that dead swimmers (zero swimming speed) accumulate in the near wall region at the edge of the capsule depleted layer, i.e., the swimmers marginate. At sufficiently high volume fractions of capsules, both the pushers and pullers demarginate, i.e., they escape from the near wall region to the channel interior. The degree of demargination increases with increasing swimming speeds.  In contrast, at low capsule volume fractions, the pushers are found to flow close to the wall irrespective of their swimming speed, while the pullers demarginate with increasing swimming speeds, just as in the case of high volume fraction suspensions. The demargination of particles is found to occur due to their collision with capsules, which may instantaneously orient them in the wall normal direction, thereby leading to their escape to the interior due to their self-propulsion provided sufficient clearance is present. The difference in demargination behavior of pushers and pullers at low volume fractions arise because the pushers tend to get attracted towards the wall, while pullers tend to get pushed away from the wall. This, coupled with the fact that at low volume fractions the capsule depleted layer thickness can be larger than the particle size, leads to the aforementioned difference in the behavior of pushers and pullers. Additional insights on the effect of size and shape of the particles will also be provided. The results discussed in this study is expected to provide useful guidelines for the design of self-propelled particles for drug delivery and other applications in blood flow.