Rotating colloids in rotating magnetic fields: Dipolar relaxation and hydrodynamic coupling.

Area of Science:

• Colloid science
• Soft matter physics
• Statistical mechanics

Background:

• Superparamagnetic colloidal particles are utilized in various applications, including microfluidics and targeted drug delivery.
• Understanding their behavior in external fields is crucial for controlling their motion and assembly.
• Previous models often simplified hydrodynamic interactions, limiting predictive accuracy.

Purpose of the Study:

• To quantitatively model the nonequilibrium dynamics of superparamagnetic colloidal particles in rotating magnetic fields.
• To compare experimental results with simulation data for validation.
• To elucidate the roles of dipolar and hydrodynamic interactions in particle dynamics.

Main Methods:

• Video microscopy (VM) was employed to observe particle behavior experimentally.
• Brownian dynamics (BD) simulations were performed to model particle motion.
• Exact two-body hydrodynamic interactions and magnetic dipole relaxation times were incorporated into the models.

Main Results:

• VM and BD results showed excellent agreement across various frequencies and amplitudes.
• Particle rotation and doublet separation were accurately predicted.
• Low-frequency rotation was dominated by dipolar interactions, while high-frequency rotation involved hydrodynamic translation-rotation coupling.

Conclusions:

• The study demonstrates the quantitative importance of precise hydrodynamic interactions and magnetic dipole relaxation for modeling Brownian colloids.
• The findings provide a validated framework for predicting the steady-state dynamics of magnetic colloids in time-dependent fields.
• This work advances the understanding of nonequilibrium phenomena in colloidal systems.