Study the fluid-solid transitions in soft colloids using particle tracking microrheology.

• Other Authors: Hua, Li.
• Format: Others
• Language: English; Chinese
• Published: 2013
• Subjects: Colloids; Surface chemistry; Particles
• Online Access: http://library.cuhk.edu.hk/record=b5549317; http://repository.lib.cuhk.edu.hk/en/item/cuhk-328745

聚（N-異丙基丙烯酸醯胺）（PNIPAM）微凝膠是一種内部有化学交联的三維網絡結構的軟膠體。這種微凝膠的物理性質是介於硬球膠體和超軟軟的星形聚合物或者膠束之間的。根據微粒的柔軟程度，微凝膠可以發生網絡互穿或者形變。因此塞滿微凝膠的體系的有效體積分數可以遠遠超過硬球系統裏的緊密堆積體積分數。這樣的系統會出現比硬球系统更加豐富的相行為。然而現在仍然缺乏對軟膠體系統在超過緊密堆積體積分數時變現的機構和性質的研究。特別是有些理論預測出的相行為仍然還沒被實際的實驗觀測的到。 === 本論文採用了一種新穎的方式去研究微凝膠體系的微流變，這種方法結合了粒子追蹤微流變和磁鑷系統。這種方法本質上是通過一種消逝波（產生於全內反射顯微鏡（TIRM）中的固液介面）作為入射光來探測靠近平直表面上事先植入的探針微粒（直徑為幾個微米）的位移。這個儀器通過記錄來自探針微粒的散射光強度來追蹤植入的探針微粒在垂直於固體水平面上的熱運動。對於位移的記錄可以達到十個納米的精度，使得它成為很靈敏的空間位置探測器。再者，通過添加了磁鑷系統，使得我們能夠有效地通過震蕩的力來在三維空間裏操控植入的微粒。通過控制探針微粒的運動，可以測量微凝膠懸浮液裏局部原位的粘彈性質。我們研究了濃度依賴和溫度依賴的PNIPAM微凝膠懸浮液的結構變化和相行為。 通過微流變的研究，讓我們第一次通過分析微凝膠懸浮液的粘彈性，確認觀測到了由有效體積分數導致的可逆轉的流體態-玻璃態-劉體態的相轉變過程。 === Soft colloids such as poly(N-isopropylamide) (PNIPAM)-based microgels are colloidal particles that consist of chemically cross-linked three-dimensional polymer networks. The physical nature of these microgel particles thus lies in between that of hard-sphere colloids, and ultrasoft star polymers as well as micelles. Due to the softness of the particles, microgels can interpenetrate or compress. As a result, microgels can be packed to effective volume fractions far above solid particles close packing, leading to the existence of much richer phase behavior when compared to simple hard colloidal particles. However, there is still a lack of knowledge on the structure and properties of soft colloid suspensions at and above close packing, and in particular some theoretically predicted phase behavior has not yet been reproduced by the experimental studies. === This thesis presents a novel approach to study the rheological properties of soft microgel suspensions using a combination of particle-tracking microrheology and magnetic tweezers. We essentially employ an evanescent wave (generated by a solid/liquid interface in the total internal reflection microscopy (TIRM)) as the incident light source to probe the displacement of an embedded probe particle (of a few micrometers diameter) near a flat surface. By measuring the scattered intensity, this technique allows tracking of the thermal motion of the embedded particle perpendicular to the solid surface to a precision of tens of nanometers, making it a highly sensitive spatial detector. Moreover, the integration of a magnetic driving force into the TIRM enables us to effectively manipulate the embedded particle in three dimensions by an oscillatory force so that the local viscoelastic properties of the microgel suspensions can be measured by resolving the particle motion. We investigated the concentration- and temperature-dependent on the structural ordering and phase behavior of PNIPAM microgel suspensions. Microrheology allows us, at a first time, to identify an effective volume fraction driven re-entrant liquid-glass-liquid phase transition by looking at the viscoelastic properties of the suspensions. We show that this result is related to the softness of the microgel particles under a confined condition, and discuss our findings in view of the existing theoretical predictions for soft particles. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Hua, Li. === Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. === Includes bibliographical references. === Abstracts also in Chinese. === 摘要 --- p.i === Abstract --- p.ii === Acknowledgement --- p.iv === Chapter 1 --- p.1 === Chapter 1.1 --- Introduction to colloids --- p.1 === Chapter 1.1.1 --- Phase transition in hard colloidal system --- p.2 === Chapter 1.1.2 --- Phase transition in soft colloidal system --- p.3 === Chapter 1.2 --- Overview of phase transition in microgels --- p.4 === Chapter 1.3 --- Simulation of soft colloids’ phase diagram --- p.8 === Chapter 1.4 --- Reference and Notes --- p.12 === Chapter 2 --- p.15 === Chapter 2.1 --- Overview of the microrheology methods --- p.15 === Chapter 2.1.1 --- Passive techniques of microrheology --- p.16 === Chapter 2.1.2 --- Active techniques for microrheology --- p.23 === Chapter 2.2 --- Microrheometer based on incorporating Magnetic Tweezers to Total Internal Reflection Microscopy --- p.29 === Chapter 2.2.1 --- Particle tracking system - TIRM --- p.30 === Chapter 2.2.2 --- Magnetic Tweezers as driven force --- p.32 === Chapter 2.2.3 --- Calibration of the magnetic force --- p.35 === Chapter 2.3 --- Reference and Notes --- p.37 === Chapter 3 --- p.44 === Chapter 3.1 --- Overview of the series of experiments --- p.44 === Chapter 3.2 --- PNIPAM microgel synthesis and characterization --- p.44 === Chapter 3.3 --- Microrheology of PNIPAM microgels suspension --- p.47 === Chapter 3.3.1 --- Volume Fraction dependence measurements --- p.49 === Chapter 3.3.2 --- Temperature depended measurements --- p.53 === Chapter 3.4 --- References and Notes --- p.55 === Chapter 4 --- p.56 === Chapter 4.1 --- Discussion and Conclusion --- p.56 === Chapter 4.2 --- Future Perspectives --- p.59 === Chapter 4.3 --- References and Notes --- p.61