Droplet microfluidics : aqueous two-phase-system droplets, microparticles and artificial motors

Abstract

Microfluidc technology has been used to synthesis functional biomaterials due to its ability to produce highly uniform droplets with tunable size, controllable structure and composition. This thesis focuses on aqueous two-phase-system (ATPS) droplets, anisotropic micropartices, and artificial motors fabrication using microfluidics. The first section is ATPS droplets generation, including a critical review, “oil-droplet-chopper” method and “transient double-emulsion” method (chapter 2, 3, 4, respectively). Recently developed ATPS droplet generation and stabilization strategies are summarized. To widen ATPS droplet generation condition and improve generation frequency, we develop an “oil-droplet chopper” method by introducing an additional dispersed oil phase. It counts on the synchronized formation of oil-in-water and ATPS droplets, where the oil droplet chops the ATPS interface into ATPS droplets. Theoretical models are developed to precisely and independently control the size and generation frequency of ATPS droplets. “Transient double emulsion”technique is developed to generate ATPS droplets on-demand. The middle oil phase is introduced to facilitate the generation of ATPS droplets and control their generation frequency and size. It involves two steps: (i) W1/O/W2 doule emulsion droplets are generated; (ii) the unstable transient double emulsion droplets then dewet into O/W2 and W1/W2 ATPS droplets. By using middle oil phase with different viscosity, the destabilization time can be tuned, and the ATPS droplets inside can be released on-demand: immediately after generation or after sustaining for a certain period. The second section is anisotropic microparticles fabrication (chapter 5). A microfluidic fiber-confined method is developed to generate anisotropic microparticles with precisely designed structure and materials compositions. Curable oil droplet-encapsulated microfibers are firstly generated, then the microfibers are dried to compress the encapsulated oil droplets into different shapes, the compressed curable oil droplets are served as templates and solidified. Finally, anisotropic microparticles with various 3D shapes are generated after dissolving the microfibers. In such way, anisotropic microparticles of different shapes, sizes and compositions can be obtained. The third section is artificial motors fabrication, including microfiber- confined method for particle-based hybrid micromotor fabrication, and direct dripping method for self-propelled all-aqueous soft milli-motor fabrication (chapter 6 and 7, respectively). Particle-baed hybrid micromotors are fabricated by adding functional materials into anisotropic microparticles, obtained from fiber-confined method. The shape and size of the micromotors, and the distribution and content of the added functional nanoparticles can be systematically and independently tailored, which are useful for micromotor’s mobility. The generated micromotors are endowed magnetic guidance and catalytic propulsion functions by dopping magnetic and Pt nanoparticles, respectively, which are capable of performing tasks of precisely catching, skilful delivery, and on-demand cargos releasing. A simple, low-cost, and efficient direct dripping method is developed for self-propelled all-aqueous soft milli-motor fabrication. By directly dripping the precursor material into the crosslinking solution, self-propelled all-aqueous soft milli-motors can be generated, which move on the water-air interface based on surface tension imbalance (Marangoni effect). The size and shape of the milli-motor can be controlled by the injection capillary dimension and injection position, respectively. By incorporating Fe and TiO2 nanoparticles, the milli-motor can be endowed with magnetical collection and waste-water treatment functions.