Fig.1 Controlled preparation of droplets, particles and capsules by microfluidics[3, 28-30]

Fig.2 Controlled preparation of droplets by microfluidics and their applications: (a) Controlled preparation of one-, two- and three-component droplets by PDMS and glass-capillary microfluidic devices[41-42]; (b) Oil-in-water food emulsions prepared by microfluidics， sunflower oil is emulsified into droplets and stabilized by soybean proteins[30]; (c) Microfluidic 3D droplet printing for the controlled preparation of droplet arrays[48]（droplet arrays are embedded in the elastomer matrix as functional units）, multi-shape, multi-mode and multi-step deformations can be achieved by different stimuli; (d) Randomly distributed droplet suspension prepared by microfludics and ordered droplet suspension prepared by microfluidic 3D droplet printing[49,51]

Fig.3 Controlled preparation of particles using single-emulsion droplets as templates and their applications: (a) Preparation of carotenoid-loaded particles by solvent evaporation and their applications as natural colorants for food[57]; (b) Preparation of cell-loaded hydrogel particles by ionic crosslinking and their applications for cell culture[59]; (c) Preparation of drug-loaded hydrogel particles by photo-triggered crosslinking and their applications for arthritis treatment[60]; (d) Preparation of stem cell-loaded porous particles by photo-triggered crosslinking and their applications for bone regeneration[61]; (e) Preparation of drug-loaded particles by chemical crosslinking and their applications for oncology therapy[29]; (f) Preparation of hydrogel particles as cell growth scaffolds by chemical crosslinking and their applications for tissue regeneration[62]

Fig.4 Controlled preparation of core-shell droplets and capsules and their applications: (a) Controlled preparation of single- and multi-core droplets by PDMS and glass-capillary microfluidic devices[42, 65]; (b) Controlled preparation of single- and multi-core droplets by microfluidic 3D droplet printing[71]; (c) Oil-core capsules prepared using single droplets as templates and by solvent evaporation[73]; (d) Hydrogel capsules with hepatocytes in the core and fibroblasts in the shell prepared using core-shell droplets as templates and by ionic crosslinking, which are used as liver organoids[75]; (e) Thin-shell water-core capsules prepared using thin-shell core-shell droplets as templates and by solvent evaporation, whose release could be triggered by osmotic pressure[76]

Fig.5 Two- and three-phase computational fluid dynamics (CFD) simulations of the formation of single droplets and core-shell droplets: (a) Two-phase CFD simulations of the droplet formation by shearing water with air[83], water is the dispersed phase and air is the continuous phase; (b) Comparison of CFD and experimental results on droplet diameter as a function of air flow rate and inner tube size[83], CFD simulations use the same parameters as those of experiments, such as inner and outer tube diameters, water viscosity, water density, water flow rate, air viscosity, air density and air flow rate; (c) Three-phase CFD simulations of the formation of core-shell droplets[84], the oil phase is the inner phase, the aqueous phase is the middle phase, and air is the continuous phase; (d) CFD results on capsule diameter, core diameter and shell thickness as a function of oil/water and water/air interfacial tensions[84], CFD simulations use the same parameters as those of experiments, such as inner and outer tube diameters, oil viscosity, oil density, oil flow rate, water viscosity, water density, water flow rate, air viscosity and air density

Fig.6 Scale-up by parallelization of microfluidic emulsification units: (a) Scale-up design based on parallel dendrites[87]; (b) Scale-up design based on divergent dendrites[88]; (c) Scale-up design based on main-branch structures[24]