Collective behavior of squirmers in confinement

Many different dynamics of motile bacteria encapsulated inside a liquid drop or a giant unilamellar vesicle has been observed, such as a co-rotating flow inside the drop and membrane tubes filled with bacteria. Experiments reported different effects from bacteria on the deformable lipid bilayer membranes in terms of vesicle net motion and deformation. In this work we investigate the collective behavior of squirmers inside two types of confinement: a rigid cell and a deformable/semi-permeable bilayer of Janus particles. Under rigid confinement we find quantitative correlations between the collective behavior and the mixtures of squirmers. Inside a vesicle with high a high bending stiffness, squirmers move around the membrane and generate a co-rotating interior fluid flow and membrane deformation at the same time. For a vesicle with a low bending stiffness, squirmers pushing the membrane can generate large deformation, such as a long tube. In extreme cases, squirmers rupture the membrane and escape, and the membrane reseal over a long time. We focus on the quantitative description of these phenomena and report the parametric dependence of interactions between squirmers and the membrane/confinement wall.