Microgels at the Oil-water Interfaces: From Fundamental Physics to Functional Materials

The adsorption phenomena of colloidal particles at the liquid-liquids interfaces have received tremendous interests in recent years. On the fundamental side, interest stems from the fact that colloidal particles confined to the interfaces can serve as an elegant system for fundamental studies of physical processes, such as two-dimensional crystallization, jamming and other phase transitions. On the practical side, interest arises as a result of demonstrated importance of the behavior of colloidal particles at the interfaces for applications in many industrial products and processes such as the production of food, anti-foam formulations, and crude oil.;In recent years, soft particles, like microgels are also employed as emulsifiers for making emulsions. These microgel particles resemble colloidal particles in many aspects. However, structurally, microgel particles constitute a three-dimensional covalently crosslinked network and can swell up in good solvents. It has been reported that emulsions stabilized by these soft microgel particles can offer an unprecedented degree of control of emulsion stability, well beyond what can be achieved by using small molecular surfactants or conventional particles. However, the stabilization and destabilization mechanism involving such soft and deformable microgels is still a matter of debate. Specifically, how microgels adsorb onto the oil-water interfaces; what parameters control the microgel adsorption; how these microgels behave at the interfaces; and how these microgels respond to environmental triggers after adsorption, are unclear.;This thesis aims at first gaining a fundamental understanding of the microgels dynamic behaviors at the oil-water interfaces, and then using this system to fabricate functional materials. This thesis contains eight parts; all of them are connected with soft microgels at the oil-water interfaces. The first part of this thesis introduces the soft microgels' performance at the oil-water interfaces. The second part focuses on the curved oil-water interfaces and the instrument we will use in this thesis. In the third part, we will present the preparations and characterizations of microgels. The fourth part addresses the microgels adsorption behaviors at the oil-water interfaces. Our results clearly show that deformability of microgel particles plays a vital role in their adsorption behaviors at the oil-water interfaces. In the fifth part, we discuss why interfacial tension (gamma) exhibits a minimum in the vicinity of PNIPAM-related microgel volume phase transition temperature (VPTT). Our results suggest that, this observed minimum can be attributed to highest deformability of microgels around VPTT as well as the interactions among the adsorbed microgels. Moreover, our results reveal that unlike conventional solid particles, the adsorbed microgels are not wetted by both oil and water. On the contrary, they will form an intruding microgel layer separating the oil and water phases, which ultimately dominates the oil-water interfacial properties. Based on the above understanding, in the sixth part, we create microgels partially covered oil-water interfaces, and investigate the microgels thermal behaviors under a confined condition. Our results show that microgels undergo an extremely slow swelling process at the oil-water interfaces. In addition, microgels would not collapse upon heating. In the seventh part of this thesis, we present the preparation of high internal phase emulsions (HIPEs) by solely using soft microgels as emulsifiers. Furthermore, we demonstrate that these microgels-stabilized HIPEs can be good templates for the preparation of hierarchical porous functional materials. Based on our investigations, in the final part, we summary the importance of microgel deformability and their interactions on microgels behaviors at the oil-water interfaces: including their adsorption dynamics, thermal-responsive behaviors, oil-water interfacial rheology properties and functional materials properties.