# Magnetic Fluids: The Interaction between the Microstructure, Macroscopic Properties, and Dynamics under Different Combinations of External Influences

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Nano-Disperse Magnetic Fluids: Discovery and Research Interest

## 3. General Information about Magnetic Fluids

_{3}O

_{4}magnetite. Fe

_{3}O

_{4}magnetite possesses a reversed spinel-type structure, and its axis of light magnetization corresponds to the diagonal of the cube (third-order axis). Table 1 provides the physical parameters of magnetite.

## 4. Physical Properties and Structure of Magnetic Fluids

#### 4.1. Polydispersity and Interparticle Interactions

**λ**represents the concentration of the solid phase

**φ**) is proposed in [79]. The highlighted areas include the following: I—Langevin superparamagnetic gas; II—interactions between MNPs can be considered using the modified effective field theory; III—the region of chain aggregate formation; IV—the area of closed chain aggregate (rings) formation; V—the area of defective structure formation; VI—the presumed area of percolating grid formation; VII—the area of unknown microstructural formations. Experimental data are available only for regions I-IV of this phase diagram. For magnetic fluids based on the magnetite, stabilized with oleic acid, and with a carrier liquid based on hydrocarbon or oil, the average value of the parameter

**λ**does not exceed one. Stable aggregates are nearly impossible in such a magnetic fluid. However, under the influence of an external magnetic field, structures can form and disintegrate when turned off. This type of magnetic fluid, associated with areas I and II, is the focus of this paper. This study’s results on the magnetic fluids structure using modern nano-analytical methods are detailed in Section 5.

#### 4.2. Features of the Magnetization of Magneto-Fluidic Systems

#### 4.3. Relaxation of Magnetic Moments of Nanoparticles in a Magnetic Fluid

#### 4.4. Magnetoviscous Effects

#### 4.5. Sound in Magnetic Fluid

#### 4.6. Stability and Aggregation of Magnetic Particles in a Magnetic Fluid in an Inhomogeneous Magnetic Field

^{6}A/м

^{2}) are described. In the experiments, the time interval required to establish concentration equilibrium was estimated. For dilute magnetic fluid (volume concentration of magnetite particles from 2% to 5%), the time to establish an equilibrium concentration distribution along the cell length of 2 mm was ~12 days.

## 5. Experimental Methods for Studying the Structure, Properties, and Dynamics of Magneto-Fluidic Systems

## 6. Controlled Active Magneto-Fluidic Systems

#### 6.1. Dynamics of Non-Magnetic Gas and Liquid Inclusions in a Magnetic Fluid

#### 6.2. Levitation of Non-Magnetic Inclusions in Magnetofluidic Systems

## 7. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**A schematic representation of the MNP in the magnetic fluid coated with a surfactant. Adapted with permission from Ref. [20]. 1998, Odenbach, S.

**Figure 2.**Dynamics of the number of publications devoted to magneto-fluidic systems, according to the Scopus database.

**Figure 3.**Influence of MNPs size on its magnetic structure and coercive force. Adapted with permission from Ref. [57]. 2010, Faraji, M.; Yamini, Y.; Rezaee, M.

**Figure 4.**Publications on the technology of magnetic fluid synthesis. Adapted with permission from Ref. [22]. 2015, Novopashin, S.A.; Serebryakova, M.A.; Khmel, S.Y.

**Figure 5.**The diagram of the orientation mechanisms of magnetic nanoparticles in a magnetic fluid: (

**a**) Néel mechanism of rotation of the magnetic moment inside the particle; (

**b**) Brown rotation of an individual particle; (

**c**) Brown rotation of the aggregate. Adapted with permission from Ref. [31]. 2014, Joseph, A.; Mathew, S.

**Figure 6.**Cryo transmission electron microscopy images of magnetic fluid samples with different particle sizes. Adapted with permission from Ref. [213]. 2003, Butter, K.; Bomans, P.H.H.; Frederik, P.M.; Vroege, G.J.; Philipse, A.P.

the density of massive magnetite ${\rho}_{f},\frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^{3}}$ [61] | 5240 |

cell parameters | a = 0.8397 nm, Z = 8 |

curie point ${T}_{C},\mathrm{K}$ [62] | 858 |

saturation of magnetization ${M}_{S},\mathrm{k}\mathrm{A}/\mathrm{m}$ | 490 |

the constant of crystallographic anisotropy $K,\mathrm{J}/{\mathrm{m}}^{3}$ | $1.1\times 1{0}^{4}$ |

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**MDPI and ACS Style**

Ryapolov, P.; Vasilyeva, A.; Kalyuzhnaya, D.; Churaev, A.; Sokolov, E.; Shel’deshova, E.
Magnetic Fluids: The Interaction between the Microstructure, Macroscopic Properties, and Dynamics under Different Combinations of External Influences. *Nanomaterials* **2024**, *14*, 222.
https://doi.org/10.3390/nano14020222

**AMA Style**

Ryapolov P, Vasilyeva A, Kalyuzhnaya D, Churaev A, Sokolov E, Shel’deshova E.
Magnetic Fluids: The Interaction between the Microstructure, Macroscopic Properties, and Dynamics under Different Combinations of External Influences. *Nanomaterials*. 2024; 14(2):222.
https://doi.org/10.3390/nano14020222

**Chicago/Turabian Style**

Ryapolov, Petr, Anastasia Vasilyeva, Dariya Kalyuzhnaya, Alexander Churaev, Evgeniy Sokolov, and Elena Shel’deshova.
2024. "Magnetic Fluids: The Interaction between the Microstructure, Macroscopic Properties, and Dynamics under Different Combinations of External Influences" *Nanomaterials* 14, no. 2: 222.
https://doi.org/10.3390/nano14020222