Many processes in microfluidics and biology are driven or affected by viscosity. While several methods are able to measure this parameter globally, very few can provide high resolution viscosity images. Optimizing the locality of viscosity measurements demands smaller probes but also shorter lateral diffusion lengths and measurement times. 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 rods. 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 from which the local viscosity is deduced. Holography provides a 3D optical field reconstruction and 3D superlocalization of the rods, which allows super-resolved viscosity measurements. Relying on the fast Brownian rotation instead of the slower translation component of nanorods therefore allows faster measurements and, crucially, smaller effective voxels for viscosity determination. We thus demonstrate that viscosity imaging is possible with a 0.5 (micron)^3 3D-resolution.

1 Sorbonne Université, CNRS, INSERM, Institut de la Vision, 17 Rue Moreau, F-75012 Paris, France. 2 Holographic Microscopy Group, Neurophotonics Laboratory, UMR 8250 CNRS, University Paris Descartes, 45 rue des Saints-Pères, 75006 Paris, France. 3 Center for Interdisciplinary Research in Biology, Collège de France, UMR 7241 CNRS, U1050 INSERM, Equipe Labellisée FRM, Paris, France. E-mail: gilles.tessier@sorbonne-universite.fr