Apparent stress-strain relationships in experimental equipment where magnetorheological fluids operate under compression mode

Abstract: This paper presents an experimental investigation of two different magnetorheological ( MR) fluids, namely, water-based and hydrocarbon-based MR fluids in compression mode under various applied currents. Finite element method magnetics was used to predict the magnetic field distribution inside the MR fluids generated by a coil. A test rig was constructed where the MR fluid was sandwiched between two flat surfaces. During the compression, the upper surface was moved towards the lower surface in a vertical direction. Stress-strain relationships were obtained for arrangements of equipment where each type of fluid was involved, using compression test equipment. The apparent compressive stress was found to be increased with the increase in magnetic field strength. In addition, the apparent compressive stress of the water-based MR fluid showed a response to the compressive strain of greater magnitude. However, during the compression process, the hydrocarbon-based MR fluid appeared to show a unique behaviour where an abrupt pressure drop was discovered in a region where the apparent compressive stress would be expected to increase steadily. The conclusion is drawn that the apparent compressive stress of MR fluids is influenced strongly by the nature of the carrier fluid and by the magnitude of the applied current

Dual Response of Materials under Electric and Magnetic Fields

The electrorheological (ER) effect is known as the change in the rheological behaviors of ER fluids under applied electric field E. When an E is imposed, ER fluids show phase transition from a liquid to a solid-like state due to the interactions of polarized particles. This solid-like behavior of particles is due to the increasing viscosity of suspensions. ER materials belong to a family of controllable fluids. ER fluids are dispersions of solid particles in a hydrophobic insulating dispersion medium. These solid particles play a very important role in the ER activity of dispersions. As the dispersed phase, diverse materials such as polymer blends, gels, biodegradable materials, clays, graphene oxide, hybrid nanocomposites, copolymers, ionic liquids, and conducting polymers have been proposed. In the magnetorheological fluids, this control is provided with magnetic field. Various magnetic particles such as carbonyl iron and iron oxides have been suggested as MR material. The combined effect of magnetic and electric field produces intensified rheological changes in the suspensions. This synergic effect is termed as electromagnetorheological effect (EMR). The EMR effect provides a new strategy to control the rheological properties of dispersions

The behaviour of magnetorheological fluids in squeeze mode

Magnetorheological (MR) fluids possess rheological properties, which can be changed in a controlled way. These rheological changes are reversible and dependent on the strength of an excitation magnetic field. MR fluids have potentially beneficial applications when placed in various applied loading (shear, valve and squeeze) modes. The squeeze mode is a geometric arrangement where an MR fluid is sandwiched between two flat parallel solid surfaces facing each other. The distance between these two parallel surfaces is called the gap size. These surfaces are either pushed towards or pulled apart from each other by orthogonal magnetic-induced forces. In this study, a test rig was designed and built to perform the experiments with three different types of MR fluids. One type of water-based and two types of hydrocarbon-based MR fluids were activated by a magnetic field generated by a coil using different magnitudes of DC electrical current. To finalize the design, a Finite Element Method Magnetics (FEMM) was used to predict the magnetic field strength throughout the MR fluids. For each trial, combination of three process parameters were experimented in both compression and tension modes on each type of MR fluid. The three process parameters were the electric current applied to the coil, the initial gap size and the compressive or tensile speed. In every test, the speed and the current in the coil were kept constant, while the instantaneous compressive and tensile forces were recorded. Experimental results showed that MR fluids have distinct unique behaviour during the compression and tension processes. The behaviour of MR fluids was dependent on the relative movement between the solid magnetic particles and the carrier fluid in both squeeze modes. A high ratio of solid particles to carrier liquid in the MR fluid is an indication of high magnetic properties. The water-based MR fluid had a relatively large solids-to-liquid ratio. At a given applied current, significant increases in compressive and tensile stresses were obtained in this fluid type. On the other hand, the hydrocarbon-based MR fluids had relatively lower solids-to-liquid ratios, whereby, less significant increases in compressive and tensile stresses were obtained. The magnetic field strength was proportional to the applied current. Consequently, the MR effect, in terms of resulting stresses, was directly proportional to the current. When plotting stress against strain for each experiment, the slopes of the curves were found to be larger in general when the initial gap sizes were smaller. This was due to higher magnetic fields generated in smaller initial gap sizes. However, the stress-strain relationships were slightly affected by changing the compressive or tensile speeds. In general, the compressive stresses were much higher than the tensile stresses for the same experimental parameters throughout this study

성능-침강안정성 상충 문제 해결을 위한 복합체 기반 자기유변유체에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2021. 2. 서용석.Magnetorheological (MR) fluids are smart materials composed of magnetic particles dispersed in magnetically-insulating carrier medium. With magnetic field, chain-like structures are formed due to dipole-induced magnetostatic interaction between magnetic particles, and the structures inhibit the flow and increase the viscosity of MR fluids in very short time. This characteristic enables the rheological properties of MR fluids to be easily tailored with magnetic field strength. Due to this unique response, MR fluids can be used for actuator systems like power steering pumps haptic devices, and active suspensions, and damper systems in automobile, bridges, buildings and so on. A huge obstacle for application of MR fluids is their poor long-term stability against the sedimentation of magnetic particles. The large difference in the density between heavy magnetic particles and light medium make magnetic particles quickly go down to bottom, reducing the MR fluids length of life. One of the strategies for improvement of the long-term stability was to reduce the density of magnetic particles by synthesizing magnetic composite materials. Fabrication of magnetic composites using light materials such as polymer, silica, carbon materials efficiently have reduced the density mismatch between magnetic materials and carrier medium, enhancing the long-term stability of MR fluids. However, there was trade-off between long-term stability and performance of MR fluids because use of light materials is equivalent to the deterioration in magnetic properties. In this study, various magnetic composites with different composition and structures were fabricated for the objective of producing MR fluids having excellent performance and long-term stability simultaneously. As a first step, hollow structured polymer-Fe3O4 composite particles were synthesized using SiO2 as sacrificial template. With cavity inside, the hollow magnetic composite particles showed the density only 40 % of bare Fe3O4 and the large improvement in long-term stability of MR suspensions could be observed. To avoid huge decline in MR performance with non-magnetic polymers, hierarchically-structured Fe3O4 nanoparticles were prepared with simple electrospraying process. By excluding polymers, hierarchically-structured Fe3O4 had magnetization value very closed to its primary nanoparticles, leading to MR performance higher more than 3 times of hollow structured polymer-Fe3O4 suspensions. At the same time, the pores inside reduced the density of the structured particles by 23 %, resulting in better long-term stability of hierarchically-structured Fe3O4 suspension than bare Fe3O4 suspension. To minimize the trade-off between MR performance and long-term stability (density of magnetic particles), non-spherical, CoFeNi-based magnetic composites were fabricated and applied for MR fluids. CNT-Co0.4Fe0.4Ni0.2 composite was produced by synthesizing Co0.4Fe0.4Ni0.2 on the surface of functionalized CNTs. Much higher magnetization of Co0.4Fe0.4Ni0.2 compared to Fe3O4 enabled CNT-Co0.4Fe0.4Ni0.2 suspension to have much superior MR performance than Fe3O4 composite-based MR fluids. Also, due to high aspect ratio of CNTs, outstanding long-term stability of 22 % light transmission was observed with formation of 3-dimensional network structures. Finally, magnetically non-active CNTs were replaced by magnetizable, flake-shaped sendust. The high drag coefficient of flake sendust, combined with roughened surface due to attached Co0.4Fe0.4Ni0.2 nanoparticles, resulted in excellent stability with 23 % of light transmission despite of the high density of sendust-Co0.4Fe0.4Ni0.2 composite particles. Also, because both constituents of sendust-Co0.4Fe0.4Ni0.2 are both magnetic materials with high magnetization value, the MR fluids retained very high yield stress value자기유변유체는 자성입자가 비자성 매개액에 분산된 현탁액 형태의 스마트물질이다. 외부 자기장 하에서 자성입자들 사이의 쌍극자로 인한 정자기성 상호작용으로 체인 형태의 구조가 형성되고, 이 구조가 유체의 흐름을 막아 매우 짧은 시간 내에 점도가 크게 향상되게 된다. 이러한 성질로 인해 자기유변유체의 유변특성을 외부 자기장을 통해 쉽게 조절하는 것이 가능하다. 이러한 외부자장에 대한 톡특한 반응성으로 인해, 햅틱 디바이스 파워스티어링 펌프, 그리고 자동차, 다리, 건물 등의 충격 방지 시스템에 자기유변유체를 이용할 수 있다. 하지만 자기유변유체의 활용은 자성입자의 침전에 대한 안정성의 부족함으로 인해 크게 제한 될 수 있다. 밀도가 높은 자성입자와 밀도가 낮은 매개액 사이의 큰 밀도차이로 인해 자성입자가 빠르게 가라앉게 되면, 자기유변유체의 수명이 크게 감소하게 된다. 이러한 문제를 해결하기 위한 한가지 방법으로 자성물질과 밀도가 낮은 물질(고분자, 실리카 탄소물질 등)을 결합하여 자성복합입자를 합성함으로써, 자성입자의 밀도를 낮추고 자기유변유체의 침강안정성을 높이는 연구들이 진행되어 왔다. 하지만 이러한 경우 복합자성입자의 자기적 성질이 저하되기 때문에 자기유변유체의 침강안정성과 성능이 서로 상충관계에 있다는 문제점을 가지고 있다. 본 논문에서는 뛰어난 성능과 침강안정성을 가지는 자기유변유체를 제조하기 위해 다양한 물질구성과 구조를 가지는 합성하였다. 첫 단계로 실리카를 템플레이트로 사용하여 할로우 구조를 가지는 고분자-Fe3O4 복합자성입자를 합성하였다. 할로우 구조 내부의 공동으로 인해, 입자의 밀도가 순수 Fe3O4 대비 40 % 수준까지 감소하였고, 이로 인해 자기유변유체의 침강안정성이 크게 상승하였다. 다음 연구로, 비자성 고분자로 인한 자기유변유체 성능의 감소를 최소화하기 위해, 간단한 전기방사 방법을 통해 계층구조를 가지는 Fe3O4 나노입자들을 제조하였다. 앞의 연구와 대비하여, 고분자의 배제를 통해 높은 자화값을 가지는 Fe3O4 나노구조입자들을 얻을 수 있었고, 이를 자기유변유체에 적용하여 할로우 고분자-Fe3O4 입자 기반 자기유변유체 대비 3배 이상의 성능을 가지는 자기유변유체를 얻을 수 있었다. 이와 동시에 Fe3O4 나노구조입자 내부에 생성된 기공들로 인해 순수 Fe3O4 대비 밀도가 약 23 % 정도 감소하였고, 이로 인해 침 Fe3O4 나노구조입자기반 자기유변유체의 침강안정성이 향상됨을 확인할 수 있었다. 자기유변유체의 성능과 침강안정성사이의 상충성을 최소화 하기 위해서 비구형의, CoFeNi 합금기반 자성 복합입자를 합성하고 자기유변유체에 적용하였다. 먼저 개질된 카본나노튜브 표면에 CoFeNi를 합성하는 방법을 통해 카본나노튜브-CoFeNi 복합체를 합성하였다. Fe3O4 대비 높은 CoFeNi의 자화값으로 인해 카본나노튜브-CoFeNi 복합체 기반 자기유변유체는 Fe3O4 복합체기반 유체 대비 3배에서 10배 이상의 뛰어난 유변성능을 보였다. 또한 종횡비가 높은 카본나노튜브로 인해 복합체가 유체 내에서 3차원 네트워크 구조를 형성하여, 빛 투과도 22 %의 매우 뛰어난 침강안정성을 보였다. 마지막으로 비자성 물질인 카본나노튜브를, 자성물질인 플레이크형 센더스트로 대체한 센더스트-CoFeNi 복합입자를 합성하여 자기유변유체에 적용하였다. 플레이크형 센더스트의 높은 종횡비로 인해 나타나는 높은 항력계수로 인해, 해당 자기유변유체는 빛 투과도 23 %의, 높은 입자밀도 대비 매우 뛰어난 침강안정성을 보였다. 동시에, 센더스트 CoFeNi 모두 높은 자화값을 가지는 자성물질이기 때문에, 센더스트-CoFeNi 기반 자기유변유체의 성능이 카본나노튜브-CoFeNi 유체 대비 크게 향상되는 것을 확인할 수 있었다.Contents Abstract ………………………………………………………..........i Contents ……………………………………………………….........v List of Tables ………………………………………………….........x List of Figures ………………………………………………..........xi Chapter 1. Introduction ……………………………………............1 1. 1. Magnetorheological (MR) Fluids and Applications………………….........1 1. 2. Long-Term Stability Problem and Proposed Solutions ……………...........4 1. 3. Research Objectives ………………………………………………….…...7 References ………………………………...........................................................9 Chapter 2. Backgrounds …………………….……………….......16 2. 1. Definition of Terms …………..................................................................16 2. 1. 1. Shear Stress …………........................................................................16 2. 1 .2. Shear Rate …………..........................................................................16 2. 1. 3. Shear Viscosity……............................................................................17 2. 1 .4. Viscoelastic Behavior .........................................................................17 2. 1. 4. 1. Storage Modulus and Loss Modulus ..........................................17 2. 2. Yield Stress of MR Fluids …………........................................................18 2. 2. 1. Rheological Models for Prediction of Dynamic and Static Yield Stress ………….............................................................................................................19 2. 2 .2. Yield Stress Dependency on the Magnetic Field Strength .................22 2. 3. Mechanism of Structures Evolution .........................................................25 References ………............................................................................................27 Chapter 3. Suspensions of Hollow Polydivinylbenzene Nanoparticles Decorated with Fe3O4 Nanoparticles as Magnetorheological Fluids for Microfluidics Applications ........31 3. 1. Introduction ………....................................................................................31 3. 2. Experimental Section ...............................................................................34 3. 2. 1. Synthesis of Hollow Polydivinylbezene (h-PDVB) Particles ............34 3. 2. 2. Deposition of Fe3O4 onto Hollow PDVB Particles ............................36 3. 2. 3. Characterization ..................................................................................37 3. 3. Results and Discussion .............................................................................41 3. 3. 1 Morphology and Structures ..................................................................41 3. 3. 2. Magnetorheological Behaviors ............................................................48 3. 3. 3. Long-Term Stability of Suspensions ...................................................62 3. 4. Conclusion ……….....................................................................................65 References ……….............................................................................................67 Chapter 4. Hierarchically Structured Fe3O4 Nanoparticles for High-Performance Magnetorheological Fluids with Long-Term Stability …………………….………………...................................74 4. 1. Introduction ………...................................................................................74 4. 2. Experimental Section ................................................................................77 4. 2. 1. Synthesis of Citric Acid-Capped Fe3O4 ..............................................77 4. 2. 2. Fabrication of HS-Fe3O4 with Electrospraying Process .....................78 4. 2. 3. Characterization ..................................................................................79 4. 3. Results and Discussion .............................................................................82 4. 3. 1. Morphology and Structures ................................................................82 4. 3. 2. Magnetorheological Behaviors ...........................................................89 4. 3. 3. Long-Term Stability of Suspensions .................................................103 4. 4. Conclusion ………..................................................................................106 References ………..........................................................................................108 Chapter 5. High-Performance Magnetorheological Fluids of Carbon Nanotube-CoFeNi Composites with Enhanced Long-Term Stability…………………….……………….................................116 5. 1. Introduction ………...............................................................................116 5. 2. Experimental Section ............................................................................119 5. 2. 1. Functionalization of Carbon Nanotubes ..........................................119 5. 2. 2. Synthesis of Co0.4¬Fe0.4Ni0.2 and CNT-Co0.4¬Fe0.4Ni0.2 .......................119 5. 2. 3. Characterization ...............................................................................120 5. 3. Results and Discussion ...........................................................................123 5. 3. 1. Morphology and Structures ..............................................................123 5. 3. 2. Magnetorheological Behaviors .........................................................130 5. 3. 3. Long-Term Stability of Suspensions .................................................142 5. 4. Conclusion ………..................................................................................145 References ………..........................................................................................146 Chapter 6. Sendust-CoFeNi Magnetic-Magnetic Composites-Based Magnetorheological Fluids for Simultaneous Improvement of Performance and Long-Term Stability ……………………..154 6. 1. Introduction ………................................................................................154 6. 2. Experimental Section .............................................................................157 6. 2. 1. Synthesis of Citric Acid-Capped Fe3O4 ...........................................157 6. 2. 2. Characterization ...............................................................................157 6. 3. Results and Discussion ...........................................................................159 6. 3. 1. Morphology and Structures ..............................................................159 6. 3. 2. Magnetorheological Behaviors ........................................................163 6. 3. 3. Long-Term Stability of Suspensions ................................................176 6. 4. Conclusion ………..................................................................................179 References ………..........................................................................................181 Chapter 7. Conclusions ………………………………………....188 7. 1. Overall conclusion ………......................................................................188 7. 2. Further works ………..............................................................................194 References ………...........................................................................................195 국문초록 ………...............................................................................................196 List of Publication ………………………………………….......199 Appendix …………………………………………..........…........201 Appendix A. Nonisothermal Crystallization Behaviors of Structure-Modified Polyamides (Nylon 6s) ...............................................................................201Docto

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

Effect of polydispersity in concentrated magnetorheological fluids

Magnetorheological fluids (MRF) are smart materials of increasing interest due to their great versatility in mechanical and mechatronic systems. As main rheological features, MRFs must present low viscosity in the absence of a magnetic field (0.1 - 1.0 Pa.s) and high yield stress (50 - 100 kPa) when magnetized, in order to optimize the magnetorheological effect. Such properties, in turn, are directly influenced by the composition, volume fraction, size, and size distribution (polydispersity) of the particles, the latter being an important piece in the improvement of these main properties. In this context, the present work aims to analyze, through experiments and simulations, the influence of polydispersity on the maximum packing fraction, on the yield stress under field (on-state), and on the plastic viscosity in the absence of field (off-state) of concentrated MRF (phi = 48.5 vol.%). Three blends of carbonyl iron powder in polyalphaolefin oil were prepared. These blends have the same mode, but different polydispersity indexes, ranging from 0.46 to 1.44. Separate simulations show that the random close packing fraction increases from about 68% to 80% as the polydispersity index increases over this range. The on-state yield stress, in turn, is raised from 30 +/- 0.5 kPa to 42 +/- 2 kPa (B ~ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8 Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the literature, increases packing efficiency and reduces the viscosity of concentrated dispersions, but beyond that, it proved to be a viable way to increase the magnetorheological effect of concentrated MRF. The Brouwers model, which considers the void fraction in suspensions of particles with lognormal distribution, was proposed as a possible hypothesis to explain the increase in yield stress under magnetic field

Yield Hardening of Electrorheological Fluids in Channel Flow

Electrorheological fluids offer potential for developing rapidly actuated hydraulic devices where shear forces or pressure-driven flow are present. In this study, the Bingham yield stress of electrorheological fluids with different particle volume fractions is investigated experimentally in wall-driven and pressure-driven flow modes using measurements in a parallel-plate rheometer and a microfluidic channel, respectively. A modified Krieger-Dougherty model can be used to describe the effects of the particle volume fraction on the yield stress and is in good agreement with the viscometric data. However, significant yield hardening in pressure-driven channel flow is observed and attributed to an increase and eventual saturation of the particle volume fraction in the channel. A phenomenological physical model linking the densification and consequent microstructure to the ratio of the particle aggregation time scale compared to the convective time scale is presented and used to predict the enhancement in yield stress in channel flow, enabling us to reconcile discrepancies in the literature between wall-driven and pressure-driven flows