  We consider a liquid droplet which is propelled solely by internal flow. In a
simple model, this flow is generated by an autonomous actuator, which moves on
a prescribed trajectory inside the droplet. In a biological system, the device
could represent a motor, carrying cargo and moving on a filamentary track. We
work out the general framework to compute the self-propulsion of the droplet as
a function of the actuating forces and the trajectory. The simplest autonomous
device is composed of three point forces. Such a device gives rise to linear,
circular or spiraling motion of the droplet, depending on whether the device is
stationary or moving along a radial track. As an example of a more complex
track we study in detail a spherical looped helix, inspired by recent studies
on the propulsion of Synechococcus1 and Myxobacteria2. The droplet trajectories
are found to depend strongly on the orientation of the device and the direction
of the forces relative to the track with the posibility of unbounded motion
even for time independent forcing.
