Continuous Frequency Sweeping Magnetorheometry Based on Variable Inductance

Abstract

Self-assembly of magnetic colloidal particles into pre-programmed microstructures plays an important role in the design of functional soft materials of interest in the microfabrication of sensors, actuators, and/or tissue engineering applications. Particle aggregates emerge because of a delicate balance between magnetic interparticle interactions and hydrodynamic interactions with the carrier. These aggregates are dynamic and follow the field when its frequency is small enough. However, for sufficiently large field frequencies the particles within the aggregates experience a time-averaged magnetic interaction that results in the formation of static (i.e., arrested) gel-like structures. The transition between the dynamic and static state is not well understood because strong fields are needed to continuously vary their frequency in a wide range. Current approaches that rely on capacitor banks require periodically turning off field generation to prevent current and voltage spikes when switching circuit relays, which disrupts the gel-like structure. These unwanted alterations in the sample’s microstructure introduce perturbations in the measurements, rendering them unreliable. In this study, we propose an inductance approach that operates at resonance, eliminating the need for switching-off periods during frequency sweeps while enabling precise control of the phase lag between the applied voltage and the generated magnetic field. This method requires continuous inductance adjustment and allows direct comparison with interrupted systems, demonstrating that field interruptions exceeding 20% duty cycle significantly affect rheological measurements. Furthermore, videomicroscopy and rheometry are employed to investigate the transition between dynamic and static behavior, as well as to monitor microstructural evolution and rheological properties under oscillatory shear. These high-precision measurements facilitate the advancement of smart and biological material design and synthesis, enhance frequency control of microrobots for medical applications, and improve both the speed and resolution of active rheometry techniques.