  Many processes in microfluidics and biology are driven or affected by
viscosity. While several methods are able to measure this parameter globally
(AFM, surface acoustic waves, DLS, ...), very few can provide high resolution
viscosity images, particularly in living cells. Here, we propose to use
sub-micrometer magnetic rods to perform high resolution viscosity imaging. An
external magnetic field forces the oscillation of superparamagnetic iron oxide
nanorods. Under linearly polarized illumination, the rotation of these highly
anisotropic optical scatterers induces a blinking which is analyzed by
heterodyne holography. The spectral analysis of the rotation dynamics yields a
regime transition frequency f t from which the local viscosity is deduced.
Holography provides 3D image reconstruction and 3D superlocalization of the
nanorods, which allows super-resolved viscosity measurements. Relying on the
fast Brownian rotation of nanorods instead of their slower translation
therefore allows faster measurements and, crucially, smaller effective voxels
for viscosity determination. Viscosity imaging is demonstrated with a 0.5
{\mu}m 3 3D resolution.
