The many behaviors of deformable active droplets

Active fluids consume fuel at the microscopic scale, converting this energy into forces that can drive macroscopic motion. In some cases, these phenomena have been well characterized, and theory can explain experimentally observed behaviors in both bulk fluids and those confined in simple stationary geometries. More recently, active fluids have been encapsulated in viscous drops or elastic shells so as to interact with an outer environment or a deformable boundary. Such systems are not as well understood. In this talk, I will discuss the behavior of droplets of an active nematic fluid. Through a mix of linear stability analysis and nonlinear simulations, we identify parameter regimes where single modes dominate and droplets behave simply: as rotors, swimmers, or extensors. When parameters are tuned so that multiple modes have nearly the same growth rate, a plethora of modes appears, including zigzaggers, washing machines, wanderers, and pulsators. The behavior of such droplets is not static, but is an emergent property of their internal dynamics and the environment that surrounds them. In particular, individual droplets in many-droplet suspensions can show qualitatively different dynamics than when isolated, as they respond to the stresses generated by those drops that surround them.