Self-propulsion and collective effects of active droplets under gravity

Active fluids exhibit complex behaviors that result from combinations of hydrodynamic, chemical, and biological effects. We explore interactions among active particles and between active particles and a solid wall through experiments and numerical simulations. In experiments, droplets of the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) are added to a concentrated surfactant solution. Solubilization of the droplets leads to self-propelling behavior and observable fluid flows around the droplets. Notably, we find that the droplets, which sediment to the bottom surface under gravity, organize into lattice-like structures. As a model for active droplets and swimming organisms, we consider neutral, extensile, and contractile spherical squirmers placed near a no-slip plane boundary and compute their trajectories in Stokes flow using boundary element method simulations. We describe transitions in their individual and collective behavior as their excess mass density is varied and compare the numerical results with experimental observations.