Magnetic dynamics of ferrofluids: mathematical models and experimental investigations

Abstract

Magnetite ferrofluids with unique magnetic behaviors are attractive for biomedical applications such as magnetic fluid hyperthermia and magnetic particle imaging. A precise nanoparticle-specific characterization by theoretical models and experiments to predict dynamics of ferrofluids and optimize their behaviors for emerging biomedical applications is necessary. In this paper, combining experiments and modeling, we have uncovered interesting magnetic dynamics of nanoparticles that are dependent on magnetic field strength, polymer coating of nanoparticles, viscosity of ferrofluid, and dipolar interactions. It is concluded that either by changing the magnitude of magnetic field or the concentrations of nanoparticles, we are able to convert the dominating relaxation process of magnetic nanoparticles from Néel to Brownian, and vice versa. Polymer coatings on nanoparticles and viscosity of ferrofluids are demonstrated to have varying degrees of influence on effective relaxation times of nanoparticles with different sizes and under different field strengths. Our theoretical models are used to predict the magnetic response of ferrofluid consisting of 35 nm magnetite nanoparticles under alternating magnetic fields, and it turns out that our theoretical data fits well with the experimental data.

Investigation of Commercial Iron Oxide Nanoparticles: Structural and Magnetic Property Characterization

“…Due to the surface spin canting effect (also called the magnetically anomalous shell or magnetically dead layer) of nanoparticles, the specific magnetizations of MNPs are always lower than that of the bulk material. Herein, the specific magnetizations of SHA series MNPs are lower than the ideal values of bulk γ-Fe 2 O 3 (60–80 emu/g) and Fe 3 O 4 (92–100 emu/g) materials. − …”

Investigating the effect of magnetic dipole–dipole interaction on magnetic particle spectroscopy: implications for magnetic nanoparticle-based bioassays and magnetic particle imaging

“…In this work, a lab-based magnetic particle spectrometer (MPS) was used to monitor the magnetic responses of SPIONs with different particle concentrations c (varying from 0.1133 nmole/mL to 3.4 nmole/mL), volume fractions f (varying from 0.001 to over 0.029), and inter-particle distances d (varying from below 78 nm to 245 nm). The MPS system setups have been reported by our previous works [31][32][33][34][35][36] and 𝑓 2 ± 4𝑓 9 were used as indicators of the physical properties of SPION suspensions. For each sample, we carried out MPS measurements at different high frequencies 𝑓 2 (400 Hz to 20 kHz).…”

“…This system consists of: a PC with LabVIEW program to control the digital acquisition card (DAQ), carry out analog to digital convertor (ADC) and discrete Fourier Transform (DFT) on the analog signals that were sent back from the pick-up coils, two instrument amplifiers (IAs) receive commands from LabVIEW and send sinusoidal waves to drive coils, two sets of drive coils to generate high and low frequency alternating magnetic fields, one pair of differentially wound pick-up coils to collect the magnetic responses from SPIONs (Faraday's law of Induction), and a plastic vial to hold SPION sample. The schematic drawings of MPS system can be found in Supporting Information S3.3.4 Experimental MethodsTwo sinusoidal magnetic fields, one with high frequency 𝑓 2 (in this work we vary 𝑓 2 from 400 Hz to 20 kHz) but low amplitude 𝐴 2 = 17 𝑂𝑒, the other with low frequency 𝑓 9 = 10 𝐻𝑧 but high amplitude 𝐴 9 = 170 𝑂𝑒 are applied to different SPION suspensions (samples i -vii)35,[37][38][39][40][41] . Under the time-varying magnetic fields, the nonlinear magnetic responses of SPIONs induce electromotive force (EMF) in the pick-up coils (Faraday's Law of Induction), which is then sent back to DAQ and PC for harmonic signal extraction (details on the signal chain can be found in Supporting Information S3).…”

One-Step, Wash-free, Nanoparticle Clustering-Based Magnetic Particle Spectroscopy Bioassay Method for Detection of SARS-CoV-2 Spike and Nucleocapsid Proteins in the Liquid Phase

“… 16 , 18 , 27 − 29 The dominant relaxation of these two processes is dependent on the magnetic core sizes of MNPs (assuming unconstrained MNPs in liquid without surface binding of any chemical substances). 30 − 32 The Brownian relaxation process is dominant for single-core iron oxide MNPs with core sizes above 20 nm (this critical size may vary for different magnetic materials of different magnetic properties). 33 − 37 This relaxation process reflects the degree of freedom of physical rotational motion of MNPs.…”