Modeling of Magnetic Particle Suspensions for Simulations

Akira Satoh

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351679008
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

The main objective of the book is to highlight the modeling of magnetic particles with different shapes and magnetic properties, to provide graduate students and young researchers information on the theoretical aspects and actual techniques for the treatment of magnetic particles in particle-based simulations. In simulation, we focus on the Monte Carlo, molecular dynamics, Brownian dynamics, lattice Boltzmann and stochastic rotation dynamics (multi-particle collision dynamics) methods. The latter two simulation methods can simulate both the particle motion and the ambient flow field simultaneously. In general, specialized knowledge can only be obtained in an effective manner under the supervision of an expert.

The present book is written to play such a role for readers who wish to develop the skill of modeling magnetic particles and develop a computer simulation program using their own ability. This book is therefore a self-learning book for graduate students and young researchers. Armed with this knowledge, readers are expected to be able to sufficiently enhance their skill for tackling any challenging problems they may encounter in future.

CHAPTER 1 General Remarks

1.1 Application fields of magnetic particle suspensions

Magnetic particle suspensions have an important potential for application and therefore a variety of studies on these functional fluids have been conducted in various fields including the traditional fluid engineering field and the recent bioengineering field where there is interest in drug delivery systems. In the field of magnetic recording material, which is a typical application field for magnetic particles, high-density recording materials [1, 2] and optical units [3–7], as shown in Fig. 1.1, have been actively studied. In these applications, suspensions composed of magnetic particles function as an intermediary medium to obtain an ultimate goal material. In another typical application field of fluid engineering, as shown in Fig. 1.2, the magneto-rheological effect, which implies that the apparent viscosity of magnetic suspensions varies due to the strength of an external magnetic and flow fields, has been applied in order to develop mechanical actuators and dampers [8, 9]. Functional particles responding to an external magnetic field, which may be developed by coating materials with magnetic particles, have a feasible potential for innovative application [10]. This kind of composite particle is designed to be controlled by an applied magnetic field whilst a non-magnetic material part of the composite particle has been synthesized to possess another specific function. In this application, the latter function constitutes the main function of the composite particle and the former function is used for transporting the particle to a specific site. Currently these types of magnetic composite particles are regarded as a key material in their application to the biomedical engineering field [11–13]. Hence, there have been vigorous attempts to synthesize multi-functionalized composite particles that can include medicines as a goal in the application to a drug delivery system [14–27], as shown in Fig. 1.3. From a suspension physics engineering point of view, the application of these higher functionalized magnetic particles to a drug delivery system is a significantly hopeful application field. From an academic point of view, it is a challenging subject to develop a technology employing a gradient magnetic field for guiding composite particles to the site of a specific cancer cell and to administer the drug agents from the particle [28–36]. Moreover, their application to the field of resource engineering and environmental engineering is also hopeful and challenging [37–39]. In these applications the precious metals or harmful substances dissolved in sea water are captured and recovered by magnetic composite particles using a gradient applied magnetic field. As clearly demonstrated in these application examples, using the characteristic feature that their physical properties and behavior in a flow field can be controlled by an external non-uniform magnetic field, functional suspensions composed of new composite magnetic particles offer significant potential for expanding their application in various fields.

Figure 1.1. Recording materials: (a) longitudinal magnetization and (b) perpendicular magnetization.

Figure 1.2. Magneto-rheological effect due to the orientational characteristics of magnetic particles in a flow field under an applied magnetic field

Figure 1.3. Application to the drug delivery system: (a) and (b) composite functional particles, and (c) a collecting method due to a gradient magnetic field.

1.2 Multi-functionalized magnetic particles

New magnetic particles (magnetic materials) such as magnetic recording materials have actively been developed by combining different kinds of molecules by many researchers [40], and representative magnetic recording materials are the FePt particles [1, 2]. In addition to the ordinary Fe₂O₃ or Fe₃O₄-based particles, there have been vigorous attempts to generate multi-functionalized magnetic particles in a variety of application fields. Such particles are synthesized by coating non-magnetic base particles with smaller magnetic particles or another magnetic coating material, which formulates a composite material made of both magnetic and non-magnetic materials with the desired characteristics for application [10, 41]. Hence, the desired behavioral characteristics of magnetic particles or composites are dependent on the field of application. In the field of fluid engineering, where the magneto-rheological effect is mainly used for applications such as mechanical dampers and actuators, we will need a suspension composed of magnetic particles that exhibits a large magneto-rheological effect and also need a mechanism to effectively control the magneto-rheological effect [8, 9]. In this application, the main factor for governing the magneto-rheological effect is the hydrodynamic interaction between the magnetic particles and an ambient flow field, therefore the shape of the magnetic particles has a significant influence on the magneto-rheological effect. Application with regard to a drug delivery system may require the most complex and highly functionalized type of magnetic particles or composites [11–13]. In this application, the drug is encapsulated in the magnetic particle or composite for transportation and guided to a specific site by an external magnetic field, and further appropriate releasing technology is necessitated to administer the drug from the encapsulated particles or composites. Hence, magnetic functional particles or composites must have several of these important characteristics for successful application in the biomedical engineering field [33–36]. Different from the ordinary synthesis technologies, where a new magnetic material is generated by altering the composition of the constituting materials (molecules), the composite magnetic particles themselves are generated by a design for obtaining the most desired characteristics as functional particles [41], as shown in Fig. 1.4, and so may lead to the appearance of characteristics that cannot be expected from the ordinary development approach. The concept of this developing approach corresponds to certain kinds of design engineering regarding mechanical machines. Hence, in addition to clarifying the physical phenomena of magnetic particle suspensions, molecular simulations have an important role in providing inspiration for the feasibility of new types of magnetic particles or composites and for stimulating the motivation for experimental researchers to generate these materials.

Figure 1.4. Various types of magnetic composite particles.

1.3 General magnetic characteristics of magnetic particles

There is a long and rich history regarding the studies of magnetism and the magnetization theory of magnetic materials in the field of magnetic material science. The studies regarding the theory of magnetism teach us that the magnetization of magnetic materials has a strong relationship with the magnetic domain structure of the materials [42, 43]: readers are recommended to refer to standard textbooks of magnetism and magnetic material if they have an interest in understanding in detail the mechanism of the magnetization process. Here we only briefly address the magnetic characteristics of magnetic particles that may be useful for synthesizing new magnetic particle suspensions.

In order to perform simulations of a magnetic particle suspension, it is necessary to clarify the magnetic properties of the dispersed magnetic particles and to idealize them in the form of a mathematical model. Since calculation of the particle-particle interactions is a significantly time-consuming task for a multi-particle system, it is desirable, from a simulation point of view, to simplify the magnetization model as much as possible. Hence, we first discuss the general magnetic characteristics of magnetic particles, which may be a v...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1. General Remarks
• 2. Forces, Energies and Torques Acting on Magnetic Particles
• 3. Modeling of Magnetic Particles for Particle-Based Simulations
• 4. Two Coordinate Systems for Description of Particle Orientation
• 5. Criterion of Particle Overlap
• 6. Particle-Based Simulation Methods
• 7. Strategy of Simulations
• 8. Description of System Characteristics
• 9. Several Examples of Simulations
• 10. Topics of Current Applications
• How to Acquire the Sample Simulation Programs
• Index