Experimental Study on the Morphological Regulation of Microfluidic Templates and Their Application in the Preparation of Lightweight Fillers

Abstract

Microfluidic technology is a systematic science and technology that processes or manipulates small amounts of fluid in microscale channels. It has been widely applied in cutting-edge fields, including biopharmaceuticals, physical and chemical analysis, medical inspection, and environmental remediation. Efficient preparation and precise control of the fluid template in the microchannel can achieve the preparation of micro-nano materials with exquisite structure and multiple components. Microfluidic templates, such as microfibers and microdroplets, have gained attention from academia and industry due to their unique hierarchical structure, rapid mass and heat transfer characteristics, and significant interfacial effects. When preparing lightweight fillers, microfluidic templates' hierarchical structure can provide a solid foundation for creating hollow structures with more precise control than traditional smelting and foaming processes. The rapid mass and heat transfer characteristics and significant interfacial effects can provide technical feasibility for further control over the morphology and size of lightweight fillers based on microfluidic templates. However, there are still few comprehensive regulation methods reported in this field. Herein, this paper focuses on the preparation of lightweight fillers based on microfluidic templates, with an emphasis on the regulation of morphology and size to address current shortcomings. The corresponding contents are listed below:

Based on the structural constraint control of low surface tension microfluidic templates, a series of co-flow capillary microfluidic systems suitable for the continuous preparation of all-aqueous microfibers were constructed. The morphology and size variations of microfibers generated under different flow conditions and structural constraints were experimentally investigated. In addition, hollow tubular silica microfibers were successfully prepared by high-temperature heat treatment processes to remove organics and sintered nanoparticles based on the as-prepared microfibers. Then, the geopolymer composites were prepared by using tubular silica microfibers as filler materials, and the control effect on the flexural properties and heat resistance of the geopolymer composites was investigated. Among them, the addition of hollow tubular silica fibers with a volume fraction of 10% can improve the flexural modulus and show better heat resistance with crack-arresting ability, which shall provide new insights for the preparation of microfiber reinforcements based on microfluidic technology and the regulation of the properties of geopolymer composites.

Based on the on-chip hydrodynamic regulation of microfluidic templates, a series of flow-focusing capillary microfluidic chip systems suitable for the continuous preparation of water-in-oil-in-water double-emulsion microdroplets were built. The core and shell dimensions, as well as their relative configuration of microdroplets generated under different flow conditions, were experimentally studied. Hollow silica microspheres were prepared through high-temperature heat treatment of water-in-oil-in-water double-emulsion microdroplets, and the effects of various heat treatment parameters on the size and surface morphology of the microspheres were systematically investigated. The mechanical properties of the microspheres were analyzed through nanoindentation tests, which showed some degree of discreteness under identical nanoindentation displacement. The maximum measured force of the sample was more than twice that of the minimum counterparts. The discrepancies identified through the finite element simulation modeling analysis could be attributed to the uneven wall thickness of hollow silica microspheres, which derived from the concentricity difference of water-in-oil-in-water double-emulsion microdroplet templates. These findings could provide a reference for the preparation of hollow microspheres as lightweight fillers using microfluidic technology.

Based on the off-chip osmosis-induced hydrodynamic regulation of microfluidic templates, a series of flow-focusing glass capillary microfluidic devices were built, which integrated the on-chip preparation of microfluidic templates and the off-chip osmosis-induced hydrodynamic regulation. The limitations of the sole on-chip hydrodynamic method for regulating microdroplets were first analyzed. Next, the effects of directional transmembrane movement of solvents induced by different osmotic pressure differences on the morphology of water-in-oil-in-water double-emulsion microdroplets were experimentally studied. Additionally, the morphological changes in the process of droplet expansion induced by different high osmotic pressure gradients were investigated, where the issue of parameter coupling in the on-chip hydrodynamic regulation could be resolved by separating the preparation and regulation process of microdroplets. As a result, hollow silica microspheres were successfully prepared through heat treatment, and the regulation effect under different osmotic pressure differences was verified through experimental observation and nanoindentation tests. Finally, geopolymer composites were prepared by model casting using hollow silica microspheres as filler materials. The mechanical and flame-retardant properties of the composites were tested accordingly. The derived results should shed light on the preparation and performance improvement of hollow microspheres based on microfluidic technology as well as the performance regulation of geopolymer composites.

In summary, we have proposed a series of novel methods to prepare microfluidic templates with well-controlled dimensions and structures, which can be further transformed into lightweight ceramic filler materials through rational heat treatment. Afterward, geopolymer syntactic foams were successfully prepared via slurry casting with the as-prepared hollow silica microfibers and microspheres as the lightweight fillers, the inclusion of which can obviously improve the properties of the composites with respect to the overall mechanical and heat resistance performances.

Date of Award	7 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pingan ZHU (Supervisor), Hongyuan JIANG (External Supervisor) & Yang Lu (External Co-Supervisor)