Enhancing magnetorheological effect using bimodal suspensions in the singlemultidomain limit

We demonstrate a new route to enhance the magnetorheological effect using bimodal suspensions in the single-multidomain limit. Experimental results are satisfactorily compared to 3D finite element method simulations. The physical reason behind this enhancement is the coating of the larger particles by the smaller ones due to the remnant magnetization of the latter.This work was supported by MAT 2016-78778-R and PCIN 2015-051 projects (FEDER FUNDS and MINECO, Spain). A J F Bombard is grateful to FAPEMIG grants: APQ-01824-17, PEE-00081-16, RED-00144-16, ETC-00043-15, PEP-00231- 15, APQ-00463-11 and RDP-00164-10. J R Morillas acknowledges FPU14/01576 fellowship

Non-linear stress response of non-gap-spanning magnetic chains suspended in a Newtonian fluid under oscillatory shear test : A direct numerical simulation

The corresponding author wishes to express his sincerest thanks to the Iran National Science Foundation (INSF) for supporting this work under Contract Number 92021291.Peer reviewedPostprintPublisher PD

Start-up rheometry of highly polydisperse magnetorheological fluids: experiments and simulations

An extensive experimental and simulation study is carried out in conventional magnetorheological fluids formulated by dispersion of mixtures of carbonyl iron particles having different sizes in Newtonian carriers. Apparent yield stress data are reported for a wide range of polydispersity indexes (PDI) from PDI = 1.63 to PDI = 3.31, which for a log-normal distribution corresponds to the standard deviation ranging from to . These results demonstrate that the effect of polydispersity is negligible in this range in spite of exhibiting very different microstructures. Experimental data in the magnetic saturation regime are in quantitative good agreement with particle-level simulations under the assumption of dipolar magnetostatic forces. The insensitivity of the yield stresses to the polydispersity can be understood from the interplay between the particle cluster size distribution and the packing density of particles inside the clusters

Magneto-Sensitive Smart Materials and Magnetorheological Mechanism

Magneto-sensitive smart materials, also named as magnetorheological (MR) materials, are a class of smart composites prepared by dispersing nanometer- or micrometer-sized ferromagnetic fillers into the different carrier matrix. As the rheological properties can be controlled by an external magnetic field rapidly, reversibly, and continuously, magneto-sensitive smart materials have great application potential in construction, automotive industry, artificial intelligence, etc. In this chapter, a brief history and classification of magneto-sensitive smart materials are firstly summarized. Next, we discuss the state-of-the-art of the magnetorheological mechanism through experimental and theoretical studies, respectively. Finally, the prospect for this material in the future is presented

Rheological Characterization of Magnetorheological Fluids

Magnetorheological (MR) fluids are smart materials that respond to an external magnetic field by changing their rheological properties. These field dependent properties are typically studied by rotational rheometry. Since the data of the rotational rheometer measurements may include errors caused by various sources, it is essential to study the rheological characterization of the MR fluids before the results are used in research. A major drawback of MR fluids is sedimentation that may weaken fluids MR response over time. The sedimentation is settling of magnetic particles induced by the large density mismatch between the particles and the carrier fluid. In this study, the rheological characterization of MR fluids by a rotational rheometer is first examined to reveal how the measurements should be done in order to gain reliable knowledge about their properties. Secondly a bidisperse size distribution of magnetic particles, consisting of micron- and nano-sized particles, was studied as a way to improve the sedimentation stability.The study of rheological characterization of MR fluids by rotational rheometer involved determination field dependent yield stresses by using various measuring procedures and plate-plate measuring geometries with different surfaces. The results demonstrated that the measured static and dynamic yield stresses are strongly affected by the plate surface characteristics as the magnetic and roughened plates provided considerably higher values than the smooth non-magnetic plates. A likely source for the difference is wall slip that may happen during rotational rheometer measurements. Furthermore, it was shown that the wall slip of MR fluid does not cause measuring gap height dependency of the results, which is surprising, as its existence is commonly used as an indicator of the wall slip.The bidisperse MR fluids were prepared by dispersing micron- and nano-sized particles in Silicone Oil (SO) or Ionic liquid (IL). The impact of the carrier fluid type and nanoparticle fraction, composition and size on the off-state viscosity, sedimentation stability and MR response were studied. The maghemite (γ − Fe2O3) nanoparticles used in the study were synthesized by Liquid Flame Spray (LFS) method that offers an interesting alternative for the chemical co-precipitation commonly used to prepare magnetic nanoparticles as it is a very versatile process. The dispersion of the micron-sized particles was better IL than in SO indicated by lower off-state viscosity and higher MR response. A partial substitution of micron-sized particles by nanoparticles provided improved sedimentation stability with both carrier fluids. The impact became stronger as the nanoparticle fraction was increased or the nanoparticle size was decreased. Both lead to higher particle surface to volume ratio and greater number of particles. These can improve the sedimentation stability by inducing higher drag between the particles and the carrier fluid or by forming a thicker nanoparticle halo around the micron-sized particles. The nanoparticles had only a minor impact on the fluids MR response, but often increased the field independent viscosity, which can be considered as a disadvantage in some applications

Rheological Characterization of Magnetorheological Fluids

Magnetorheological (MR) fluids are smart materials that respond to an external magnetic field by changing their rheological properties. These field dependent properties are typically studied by rotational rheometry. Since the data of the rotational rheometer measurements may include errors caused by various sources, it is essential to study the rheological characterization of the MR fluids before the results are used in research. A major drawback of MR fluids is sedimentation that may weaken fluids MR response over time. The sedimentation is settling of magnetic particles induced by the large density mismatch between the particles and the carrier fluid. In this study, the rheological characterization of MR fluids by a rotational rheometer is first examined to reveal how the measurements should be done in order to gain reliable knowledge about their properties. Secondly a bidisperse size distribution of magnetic particles, consisting of micron- and nano-sized particles, was studied as a way to improve the sedimentation stability.The study of rheological characterization of MR fluids by rotational rheometer involved determination field dependent yield stresses by using various measuring procedures and plate-plate measuring geometries with different surfaces. The results demonstrated that the measured static and dynamic yield stresses are strongly affected by the plate surface characteristics as the magnetic and roughened plates provided considerably higher values than the smooth non-magnetic plates. A likely source for the difference is wall slip that may happen during rotational rheometer measurements. Furthermore, it was shown that the wall slip of MR fluid does not cause measuring gap height dependency of the results, which is surprising, as its existence is commonly used as an indicator of the wall slip.The bidisperse MR fluids were prepared by dispersing micron- and nano-sized particles in Silicone Oil (SO) or Ionic liquid (IL). The impact of the carrier fluid type and nanoparticle fraction, composition and size on the off-state viscosity, sedimentation stability and MR response were studied. The maghemite (γ − Fe2O3) nanoparticles used in the study were synthesized by Liquid Flame Spray (LFS) method that offers an interesting alternative for the chemical co-precipitation commonly used to prepare magnetic nanoparticles as it is a very versatile process. The dispersion of the micron-sized particles was better IL than in SO indicated by lower off-state viscosity and higher MR response. A partial substitution of micron-sized particles by nanoparticles provided improved sedimentation stability with both carrier fluids. The impact became stronger as the nanoparticle fraction was increased or the nanoparticle size was decreased. Both lead to higher particle surface to volume ratio and greater number of particles. These can improve the sedimentation stability by inducing higher drag between the particles and the carrier fluid or by forming a thicker nanoparticle halo around the micron-sized particles. The nanoparticles had only a minor impact on the fluids MR response, but often increased the field independent viscosity, which can be considered as a disadvantage in some applications

Rheological and stability properties of magnetorheological fluid with superparamagnetic maghemite nanoparticles

This research is focused on the development of a new magnetorheological (MR) fluid which contains maghemite (γ-Fe2O3) nanoparticles so as to improve its performance. The performance of MR fluid is presented in terms of physical and rheological properties and its application in MR device. In this work, the γ-Fe2O3 has been synthesized using co-precipitation method and coated with oleic acid. Two types of MR fluids were prepared, bidisperse MR fluid containing carbonyl iron (CI) microparticles substituted with γ-Fe2O3 and MR fluid utilizing γ-Fe2O3 additive. MR fluid containing γ-Fe2O3 showed great improvement exhibiting reduced sedimentation rate and enhanced re-dispersibility. During the period of 50 hours, the bidisperse MR fluid with 5 wt% of γ-Fe2O3 reduced 15% of sedimentation rate and MR fluid with 1 wt% of γ-Fe2O3 additive reduced 9.6% of sedimentation rate compared to pure CI MR fluid. The rheological properties of the MR fluid were analyzed with respect to the rheological models of Bingham Plastic, Herschel Bulkley and Casson models. The rheological properties of bidisperse MR fluid revealed that the substitution of 5 wt% γ-Fe2O3 increased the yield stress by 8.5% but further substitution of γ-Fe2O3 would slightly decrease the yield stress. On the other hand, the MR fluid added with γ-Fe2O3 additive showed improvement in yield stress over the entire range of magnetic field applied. The results indicated that the addition of 1 wt% of γ-Fe2O3 in MR fluid increased the yield stress by 11.7%. The performance of MR fluid using MR valve equipped with a hydraulic bypass damper resulted in improvement of damping force when γ-Fe2O3 is added. The MR fluid with 1 wt% γ-Fe2O3 additive improved the maximum damping force up to 11.1% compared to the pure MR fluid. Therefore, the substitution and addition of γ-Fe2O3 nanoparticles in the MR fluid improved both its physical and rheological properties, hence it can potentially be used in commercial application as a simple and reliable damping device

침강 안정성이 향상된 고성능 자기유변유체에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2020. 8. 서용석.Magnetorheological (MR) fluids are typically consist of magnetic particles (Carbonyl Iron, Fe2O3, Fe3O4 and so on) in a magnetically insulating fluid (water, silicon oil and so on). When a magnetic field induces attractive interactions between the magnetic particles, these particles form a solid-like network of fibril shapes within a few milliseconds oriented along the direction of the magnetic field. Reverse transition occurs as soon as the magnetic field is switched off. These features lead to remarkable changes in the rheological properties of the fluid which shows wide potential applications such as dampers, brakes, shock observers, drug delivery, and robotics, etc and could be controlled by adjusting the strength of the magnetic field depending on applications. Despite substantial advanced in commercialization, MR fluids have long-term stability issues that significantly limit their usefulness and also need to be predicted the precise flow behavior. In this thesis, we propose the constitutive equation to predict the flow behavior of MR fluid and investigate a number of MR fluid composed of soft-magnetic composite particles to overcome the sedimentation drawback. Firstly, as modeling and analysis are essential to optimize material design, describe the flow behavior over a wide range of shear rate and distinguish between static yield stress and dynamic yield stress, the precise knowledge of the relationships between the suspension rheological properties and such variables as the deformation rate, the applied magnetic field strength, and the composition are required. So we re-analyze the constitutive equation proposed before to describe the MR fluids flow and propose new constitutive equation. The proposed Seo-Seo model predicted the flow behavior precisely compared to pre-exist constitutive model and also yielded a quantitatively and qualitatively precise description of MR fluid rheological behavior based on relatively few experimental measurements. To overcome sedimentation drawback, the core/shell structured Foamed polystyrene/Fe3O4 Particles were synthesized by applying a dual-step processing comprising pickering emulsion polymerization, subsequently by the foaming of polystyrene core using the supercritical carbon dioxide fluid foaming process. Through these processes, the density of composite was dropped significantly and the long-term stability was improved. As polystyrene located core part and magnetic particle contact directly, the magnetorheological properties of the Foamed polystyrene/Fe3O4 were considerable compared to pure Fe3O4. Even though the core/shell structured Foamed polystyrene/Fe3O4 showed considerable level, the magnetorheological properties got worsen because polystyrene is magnetically non-active. So, we synthesized hollow shape Fe3O4 particles without any magnetically non-active template. As a result, compared to the core/shell structured Foamed polystyrene/Fe3O4, the density of hollow shape Fe3O4 particles rise slightly and the magnetorheological properties reached outstanding level, and the long-term stability maintained. Also, the conformation of solid-like network of fibril shapes changes were investigated by using micro/nano size Fe3O4 particles to verify the reinforcement effect. As the particle size increases, the magnetorheological properties improve due to a rise of the magnetic saturation level. However, depending on the ratio of the nano size Fe3O4 particles, an overturning of the magnetorheological properties and the magnetic saturation was observed. This phenomenon is because of the cavity among the micro size Fe3O4 particles. The micro size Fe3O4 particles develops a relatively coarse solid-like network of fibril shapes. The chain conformation of a bidisperse MR fluid shows quite different from that of the micron size Fe3O4 particles-based fluids. The nano size Fe3O4 particles appear to fill in the cavity among the micro size Fe3O4 particles. As a result, this distinct conformation reinforced the magnetorheological properties. Finally, the shape effect of the magnetic particle on magnetorheological properties and sedimentation stability was investigated by using two types of sendust which are bulk and flake type. The flake type sendust has a small demagnetization factor because its domain orients one direction. This feature lead to extraordinary behavior which is a rapid transition to solid-like network at low magnetic field. Also, its high aspect ratio leads to a large drag coefficient which improve the long-term stability.자기유변유체는 물 또는 비수계(실리콘 오일 등)의 유체에 자화 가능한 미세입자(철 마이크로 입자)를 분산시킨 현탁액으로서, 외부로부터 제공되는 강한 자기장에 따라 짧은 시간안에 탄성, 소성, 점도 같은 자기유변효과를 나타내는 유체를 말한다. 자기유변유체는 외부 자기장에 의해 유변효과를 조절할 수 있기 때문에 다양한 응용분야로의 적용 가능성에 대한 관심이 증가하고 있다. 그러나 자성입자와 현탁 유체와의 밀도 차에 의해 발생하는 침전현상으로 인해 자기유변유체의 실제적인 응용이 제한되고 있다. 본 연구에서는 자기유변유체의 거동을 예측하는 구성방정식을 제안하고, 침전 문제를 극복하기 위해 연자성 복합체로 구성된 자기유변유체를 조사한다. 재료 설계를 최적화하기 위해 필수적으로 광범위한 전단 속도에 걸친 흐름 동작을 설명하고 정적 항복 응력과 동적 항복 응력을 구분하여야 한다. 또한, 현탁액의 유전학적 특성과 변형률, 적용된 자기장 강도 및 구성과 같은 변수 사이의 관계에 대한 정확한 지식이 필요하다. 따라서, 자기유변유체의 흐름을 설명하기 위한 기존의 제안된 구성방정식을 분석하고 새로운 구성방정식을 제안한다. 새롭게 제안한 구성 방정식인 서-서 모델은 기존에 존재하는 구성방정식과 비교하여 유체의 흐름을 정확하게 예측하였고, 비교적 적은 실험 값을 바탕으로 자기유변유체의 흐름에 대한 정량적, 질적으로 정밀한 설명을 도출하였다. 침전 문제를 극복하기 위해 피커링 에멀전 중합을 및 초임계 이산화탄소를 이용한 발포공정의 이중 공정 처리를 통해 코어-쉘 구조의 발포 스타이렌 고분자-철 복합체를 합성하였다. 이중 공정 처리를 통해 복합체의 밀도가 현저히 떨어지고 장기 안정성이 향상되었다. 또한, 스타이렌이 코어 부분에 위치하여, 철 입자가 직접적인 접촉을 통해 높은 자력 특성을 얻었다. 코어-쉘 구조의 발포 스타이렌 고분자-철 복합체의 자력 특성이 상당한 수준을 보였음에도 불구하고, 스타이렌이 자력적으로 비활성화 물질이므로 순수한 철에 비해 자력 특성은 약화되었다. 따라서 자력적으로 비활성화 물질인 스타이렌을 제거하여 지지대가 없는 중공형상의 철 입자를 합성하였다. 그 결과, 코어-쉘 구조의 발포 스타이렌 고분자-철 복합체에 비해 중공형상의 철 입자는 밀도가 약간 상승하였으나 높은 자력특성을 보였고 장기 안정성이 유지되었다. 추가적으로 마이크로/나노 크기의 철 입자를 사용하여 피브릴 구조의 보강효과를 검증하였다. 입자 크기가 증가함에 따라 자기 포화 수준의 상승으로 자력특성이 개선되었다. 그러나, 나노 크기의 철 입자의 비율에 따라 자력특성과 자기 포화 현상의 역전현상이 관찰되었다. 이 현상은 마이크로 크기의 철 입자의 피브릴 구조를 형성시에 철 입자 사이의 공동때문이다. 마이크로 크기의 철 입자는 비교적 거친 피브릴 구조를 형성한다. 혼성 자기유변체는 마이크로 크기의 철 입자와는 다른 피브릴 구조를 형성한다. 나노 크기의 철 입자들이 마이크로 크기의 철 입자 사이의 공동을 채움으로 인해서 자력특성이 향상되었다. 마지막으로, 벌크형과 박리형의 센더스트를 이용하여 자기입자의 모양이 유변적 특성에 끼치는 영향을 조사하였다. 박리형 센더스트의 자구는 한 방향으로 정렬되어 있어 작은 감자율을 갖고, 이 특징은 저자기장에서 피브릴 구조로의 빠른 전환을 가능하게 한다. 또한, 박리형 센더스트의 높은 종횡비로 인한 항력계수는 장기 안정성을 향상시켰다.Chapter 1. Introduction and Background . 0 1.1. Magnetorheological (MR) Fluids 0 1.2. Applications of MR fluids . 2 1.3. Rheology 2 1.3.1. Flow behavior . 3 1.3.1.1. Definition of terms 3 1.3.1.1.1. Shear stress 5 1.3.1.1.2. Shear rate 5 1.3.1.1.3. Shear viscosity . 5 1.3.1.2. Flow and viscosity curve 7 1.3.1.2.1. Ideal viscous flow. 7 1.3.1.2.2. Shear-thinning flow and Shear-thickening . 9 1.3.1.2.3. Yield stress 9 1.3.2. Viscoelastic behavior 11 1.3.2.1. Storage modulus and Loss modulus . 11 Reference 12 Chapter 2. Constitutive Equation . 14 2.1. Introduction . 14 2.2. Rheological Models for the Yield Stress . 18 2.2.1. Static Yield Stress versus Dynamic Yield Stress . 18 2.2.2. Yield Stress Dependency on the Magnetic Field Strength 22 2.2.3. Mechanism of Structure Evolution . 24 2.3. Conclusion . 26 Reference . 27 Chapter 3. High-Performance Magnetorheological Suspensions of Pickering Emulsion Polymerized Polystyrene/Fe3O4 Particles with Enhanced Stability 31 3.1. Introduction 31 3.2. Experimental Section 33 3.2.1. Synthesis of Polystyrene/Fe3O4 particles . 33 3.2.2. Synthesis of Foamed Polystyrene/Fe3O4 particles 34 3.2.3. Characterization 37 3.3. Results and Discussion 41 3.3.1 Morphology . 41 3.3.2. Magnetorheological Behaviors . 42 3.3.3. Yield Stress of the MR Fluids 47 3.3.4. Structure Evolution Mechanism and the Suspension Stability . 54 3.4. Conclusion . 59 References . 61 Chapter 4. Template Free Hollow Shaped Fe3O4 Micro-Particles for Magnetorheological Fluid . 65 4.1 Introduction . 65 4.2. Experiment Section . 67 4.2.1. Synthesis of Fe3O4 particles (Pure Fe3O4) . 67 4.2.2. Synthesis of PS/Fe3O4 particles (Picker) . 68 4.2.3. Synthesis of PS/Fe3O4@Fe3O4 particles (C-picker) 68 4.2.4. Synthesis of templet free hollow shaped Fe3O4 (H-Picker) . 69 4.2.5. Characterization 69 4.3. Results and Discussion . 70 4.3.1. Particle Morphologies and Magnetic Hysteresis Curve 70 4.3.2. Magnetorheological Behaviors . 76 4.3.3. Yield Stress of the MR Fluids . 80 4.3.4. Mechanism of Structure Evolution and Suspension Stability . 84 4.4. Conclusion 89 Reference 90 Chapter 5. Bidisperse MR Fluids Using Nano/micro Size Fe3O4 particles . 95 5.1. Introduction 95 5.2. Experiment Section 99 5.2.1. Material. 99 5.2.2. Characterization Methods . 99 5.3. Results and Discussion . 99 5.4. Conclusion 106 References . 107 Chapter 6. Shape effect of magnetic particle on magnetorheological (MR) properties and sedimentation stability 108 6.1. Introduction . 108 6.2. Experiment Section . 109 6.2.1. Material . 109 6.2.2. Characterization Methods 109 6.3. Results and Discussion 110 6.3.1. Particle Morphologies and Magnetic Hysteresis Curve . 110 6.3.2. Magnetorheological Behaviors 116 6.3.3. Yield Stress of the MR Fluids . 120 6.3.4. Mechanism of Structure Evolution and Suspension Stability . 124 6.4. Conclusion . 129 References 130 Chapter 7. Conclusions 135 국문초록 139 List of Publication 141 Appendix . 142 Appendix A. Improvement of Mechanical Properties by Introducing Curable Functional Monomers in Stereolithography 3D PrintingDocto

Chain formation and phase separation in ferrofluids: The influence on viscous properties

Ferrofluids have attracted considerable interest from researchers and engineers due to their rich set of unique physical properties that are valuable for many industrial and biomedical applications. Many phenomena and features of ferrofluids' behavior are determined by internal structural transformations in the ensembles of particles, which occur due to the magnetic interaction between the particles. An applied magnetic field induces formations, such as linear chains and bulk columns, that become elongated along the field. In turn, these structures dramatically change the rheological and other physical properties of these fluids. A deep and clear understanding of the main features and laws of the transformations is necessary for the understanding and explanation of the macroscopic properties and behavior of ferrofluids. In this paper, we present an overview of experimental and theoretical works on the internal transformations in these systems, as well as on the effect of the internal structures on the rheological effects in the fluids. © 2020 by the authors.The research was carried out within the state assignment of the Ministry of Science and Higher Education of the Russian Federation, (theme “Magnet,” contract no. 02.A03.21.006 and project no. FEUZ-2020-0051)