  Artificial micro and nano machines have been envisioned and demonstrated as
potential candidates for variety of applications, ranging from targeted drug or
gene delivery, cell manipulation, environmental sensing and many more. Here, we
demonstrate the application of helical nanomachines that can measure and map
the local rheological properties of a complex heterogeneous environment. The
position of the helical nanomachine was controlled precisely using magnetic
fields, while the instantaneous orientation provided an estimation of the
viscosity of the surrounding medium with high spatial and temporal accuracy.
Apart from providing viscosity estimates in purely viscous and viscoelastic
media with shear rate independent viscosity (Boger fluids), their motion was
also found to be extremely sensitive to fluid elasticity. Taken together we
report a promising new technique of mapping the rheological properties of a
complex fluidic environment by helical nanomachines with high spatial and
temporal resolutions, a functionality that goes beyond the capabilities of
existing passive and active microrheological methods.
