Vesicle Transformations under Swim Pressure

All types of living cells and their organells are bound by membranes, which are mostly immersed in active environments. In active matters, an important quantity swim pressure was recently shown to depend on boundary shapes. Consequently, understanding how swin pressure interplays with fluidic membranes is highly desired. For the first time, we study vesicle transformations under swim pressure exerted by filled self-propelled particles. Our simplest model gives a novel phase diagram for vesicle shapes with a variation of vesicle volumes and particle driving speeds. Moreover, we surprisingly find a first order phase transition between spherical and prolate shapes without volume constraint. The origin of this phase transition is then well revealed by a theoretical calculation for single particle. Our predictions are directly verifiable in experiments on vesicles filled with Pt-coated Janus particles. Furthermore, they can help to understand and exploit various active behaviors of membranes in both living and designed matters.