Activity driven droplet shape fluctuations and motility

The presence of active particles, such as bacteria or motile colloids, within a soft confinement changes both the collective behaviors of the particles and the shape dynamics of the confinement. We model and simulate active particles encapsulated in 2D droplet, whose boundary is modeled as a ring of spring connected beads. We explore the behavior of both the active particles and the confinement to identify their effects on each other. The collision of the active particles with the droplet boundary and their hydrodynamic interactions lead to the formation of a collective vortex motion inside the droplet as observed in experiments [1]. The power spectrum of the amplitude of the shape fluctuations, induced by the collision of the active particles, show a power spectrum decay of q^{-4}, which differs from the q^{-2} decay expected from purely thermal surface tension controlled fluctuations. The addition of random reorientation to mimic run and tumble bacteria, shows the active particles no longer form a vortex inside the confinement, although the power spectrum remains unchanged. The particle activity also gives rise to super-diffusive net motion of the droplet.



[1] Kokot, G., Faizi, H.A., Pradillo, G.E., Snezhko, A. and Vlahovska, P.M., 2022. Spontaneous self-propulsion and nonequilibrium shape fluctuations of a droplet enclosing active particles. Communications Physics, 5(1), pp.1-7.