Microchannel droplet formation mechanism

Spontaneous Droplet Formation in Microchannels This phenomenon was originally demonstrated by Kawakatsu and coworkers [7], and the droplet formation mechanism was first proposed by... 

By microchanneling (Figure 20.8, left), monodisperse emulsions may be produced in the absence of shear forces [63]. A strongly non-cylindrical geometry at the microchannel exit followed by a terrace is responsible for this effect [64, 65]. The droplet formation mechanism is limited to a critical velocity of the dispersed phase, above which the droplet sizes are widely distributed [51]. 

Figure 13.19 Schematic droplet formation mechanism in microchannel emulsification [18].

In microchannel emulsification the mechanism of droplet formation is totally different. Here, single droplets are formed in the absence of any shear forces. 

Microfluidic generation of polymer particles starts from the emulsiflcation of liquid monomers, oligomers, or polymers. This step is followed by polymerization, cross-linking, or physical gelation of the molecules compartmentalized in droplets. Dimensions of polymer microbeads are predetermined by the dimensions of precursor droplets, which are in turn controlled by the geometry and dimensions of microchannels in MF droplet generator, the mechanism of droplet formation, the macroscopic properties of the droplet and continuous phases, and the relative volumetric flow rates of the continuous and droplet phases. 

The mechanisms of formation of discrete segments of fluids in microfiuidic flow-focusing and T-junction devices, that we outlined above point to (i) strong effects of confinement by the walls of the microchannels, (ii) importance of the evolution of the pressure field during the process of formation of a droplet (bubble), (iii) quasistatic character of the collapse of the streams of the fluid-to-be-dispersed, and (iv) separation of time scales between the slow evolution of the interface during break-up and last equilibration of the shape of the interface via capillary waves and of the pressure field in the fluids via acoustic waves. These features form the basis of the observed - almost perfect -monodispersity of the droplets and bubbles formed in microfiuidic systems at low values of the capillary number. 

Another important application of the front-tracking method is to simulate the drop/bubble formation in flow-focusing devices. Production of mono disperse drops ubbles in microchannels is of fundamental importance for the success of the concept of lab-on-a-chip. It has been shown that flow-focusing can be effectively used for this purpose. Filiz and Muradoglu performed front-tracking simulations in order to understand the physics of the breakup mechanism and effects of the flow parameters on the droplet/ bubble size in the flow-focusing devices [11]. 

N ew opportunities and future directions in the area of microchannel emulsification are most likely in the areas of scale-up [140,141], encapsulation/polymeriza-tion [123, 125, 158, 164—169], rapid quenching of droplets [135], and the use of emulsions as templates for uniform macroporous particle structure formation [172]. MicroChannel emulsification is also likely to open up new opportunities with systems that are highly shear-sensitive [120, 135, 173]. The ability to scale up the process will spur new markets that require high production rates and the production of monodisperse capsules and polymer particles. Such developments will be useful in drug delivery applications and will contribute to the further quantification of micro-particle properties. Additionally, the use of monodisperse emulsions as particle templates is likely to enhance the utility of highly functional nanoparticles in need of a deployment mechanism [172]. 

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