Nanoscale rheometry of viscoelastic soft matter by oscillating field magneto-optical transmission using ferromagnetic nanorod colloidal probes

Nickel nanorods with an average length of 250-420 nm and diameter of 20-26 nm were prepared by pulsed current electrodeposition into porous aluminum oxide templates and dispersed as colloidal probes in water-based viscoelastic matrices. The ferromagnetic single domain nanorods were driven to rotational motion by an oscillating magnetic field. Nanorod rotation was detected using optical transmission of linearly polarized light providing a frequency-dependent complex magneto-optical response function. Quantitative data analysis was derived for the two most basic mechanical equivalents to viscoelastic materials, the Voigt-Kelvin and Maxwell model, respectively, and demonstrated by means of two examples. The transition from a viscous fluid towards a viscoelastic hydrogel with static shear elasticity was monitored by analyzing an isothermal series of magneto-optical measurements of a gelatin sol after temperature quench in terms of the Voigt-Kelvin model. Maxwell-type relaxation was investigated using CTAC/NaSal giant wormlike micellar solution as matrix. The viscosities and shear moduli retrieved from magneto-optical measurements were compared with macroscopic values obtained by conventional shear rheometry. Characteristic features for each model system were found in the rheological properties at both length scales, yet with quantitative differences caused by the small size of the nanorod probe particles. (C) 2014 AIP Publishing LLC.