Rectification of confined soft vesicles containing active particles.

One of the most promising features of active systems is that they can extract energy from their environment and convert it to mechanical work. Self propelled particles enable rectification when in contact with rigid boundaries. They can rectify their own motion when confined in asymmetric channels and that of microgears. In this paper, we study the shape fluctuations of two dimensional flexible vesicles containing active Brownian particles. We show how these fluctuations not only are capable of easily squeezing a vesicle through narrow openings, but are also responsible for its rectification when placed within asymmetric confining channels (ratchetaxis). We detail the conditions under which this process can be optimized, and sort out the complex interplay between elastic and active forces responsible for the directed motion of the vesicle across these channels.