Magnetic sub-micron rods for quantitative viscosity imaging using heterodyne holography

TL;DR

This paper introduces a high-resolution viscosity imaging method using magnetic sub-micron rods and heterodyne holography, enabling 3D viscosity maps with sub-micron spatial resolution in microfluidic and biological samples.

Contribution

The study presents a novel technique combining magnetic nanorods and heterodyne holography for super-resolved, high-speed viscosity imaging with 0.5 cubic micron resolution.

Findings

Achieved 3D viscosity imaging with 0.5 μm³ resolution.

Demonstrated faster measurements using rotational Brownian motion.

Enabled superlocalization of magnetic rods for precise viscosity mapping.

Abstract

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.