12,003 research outputs found

    Thermostating by Deterministic Scattering: Heat and Shear Flow

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    We apply a recently proposed novel thermostating mechanism to an interacting many-particle system where the bulk particles are moving according to Hamiltonian dynamics. At the boundaries the system is thermalized by deterministic and time-reversible scattering. We show how this scattering mechanism can be related to stochastic boundary conditions. We subsequently simulate nonequilibrium steady states associated to thermal conduction and shear flow for a hard disk fluid. The bulk behavior of the model is studied by comparing the transport coefficients obtained from computer simulations to theoretical results. Furthermore, thermodynamic entropy production and exponential phase-space contraction rates in the stationary nonequilibrium states are calculated showing that in general these quantities do not agree.Comment: 16 pages (revtex) with 9 figures (postscript

    Optofluidic fabrication for 3D-shaped particles.

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    Complex three-dimensional (3D)-shaped particles could play unique roles in biotechnology, structural mechanics and self-assembly. Current methods of fabricating 3D-shaped particles such as 3D printing, injection moulding or photolithography are limited because of low-resolution, low-throughput or complicated/expensive procedures. Here, we present a novel method called optofluidic fabrication for the generation of complex 3D-shaped polymer particles based on two coupled processes: inertial flow shaping and ultraviolet (UV) light polymerization. Pillars within fluidic platforms are used to deterministically deform photosensitive precursor fluid streams. The channels are then illuminated with patterned UV light to polymerize the photosensitive fluid, creating particles with multi-scale 3D geometries. The fundamental advantages of optofluidic fabrication include high-resolution, multi-scalability, dynamic tunability, simple operation and great potential for bulk fabrication with full automation. Through different combinations of pillar configurations, flow rates and UV light patterns, an infinite set of 3D-shaped particles is available, and a variety are demonstrated

    A Generalised Model of Investment under Uncertainty: Aggregation and Estimation

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    We propose a structural model of investment which is based on the aggregation of (S,s) investment projects within firms. This encompasses the findings that whilst firm level investment is smooth, plant level investment is lumpy and frequently zero. We undertake stochastic aggregation and derive a structural firm level investment estimator. The empirical performance and fit of this estimator on a panel of manufacturing firms is encouraging and provides an avenue for general policy simulation. This model also explains the rich non-linear dynamics of firm level investment data and the frequent simultaneity of firm level investment and disinvestment. This approach provides an alternative structural estimator to the standard convex adjustment cost models, such as Tobin's Q and the Euler equation. The is important because these estimators, which assume quadratic adjustment costs, appear to be misspecified and subject to a fallacy of composition between smooth firm level investment and lumpy plant level investment. For completeness we also consider time aggregation as an alternative source of smoothing but statistically reject this as being insufficient to smooth investment alone. This test also rejects most plant level data, such as the US\ LRD and UK\ ARD, as being generated from a single (S,s) process.

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

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    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
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