  A wide range of microorganisms, e.g. bacteria, propel themselves by rotation
of soft helical tails, also known as flagella. Due to the small size of these
organisms, viscous forces overwhelm inertial effects and the flow is at low
Reynolds number. In this fluid-structure problem, a competition between elastic
forces and hydrodynamic (viscous) forces leads to a net propulsive force
forward. A thorough understanding of this highly coupled fluid-structure
interaction problem can not only help us better understand biological
propulsion but also help us design bio-inspired functional robots with
applications in oil spill cleanup, water quality monitoring, and infrastructure
inspection. Here, we introduce arguably the simplest soft robot with a single
binary control signal, which is capable of moving along an arbitrary 2D
trajectory near air-fluid interface and at the interface between two fluids.
The robot exploits the variation in viscosity to move along the prescribed
trajectory. Our analysis of this newly introduced soft robot consists of three
main components. First, we fabricate this simple robot and use it as an
experimental testbed. Second, a discrete differential geometry-based modeling
framework is used for simulation of the robot. Upon validation of the
simulation tool, the third part of this study employs the simulations to
develop a control scheme with a single binary input to make the robot follow
any prescribed path.
