Flow Focusing Devices

Garstecki, P., Gitlin, I., DiLuzio, W., Whitesides, G.M., Kumacheva, E. and Stone, H.A. (2004). Formation of monodisperse bubbles in a microfluidic flow-focusing device. Appl Phys. Lett. 85,2649-2651. 

Figure 4. The T-junction [9] (Adapted from Ref [13]). An axisymmetric flow-focusing geometry [10] (Adapted from Ref [14]). A planar flow-focusing device [12] (Adapted from Ref [15]).

Figure 5. The simple periodic mode of a flow-focusing device illustrated on five micrographs taken during the period of formation of a single bubble in this microfluidic flow-focusing device [15].

Figure 6. Experiments on time-resolved tracking of the shape of the gas-liquid interface during the process of formatiorr of a single bubble in a microfluidic flow-focusing device. From a video recording of the process of break-up, we extract the projection of the interface on the x-y plarre (plane of the microfluidic device). We then extract the minimum width of the neck as a functiorr of time (Adapted Ifom Ref [21]).

Figure 9. Formation of droplets in microfluidic flow-focusing devices. The micrographs on the left illustrate the process of formation of aqueous drops in an organic continuous fluid [S. Makulska, P. Garstecki, Institute of Physical Chemistry PAS]. The chart on the right shows the dependence of the volume of liquid droplets formed in a planar flow focusing device on the value of the capillary number (Adapted from Ref [24]).

B. Dollet, W. van Hoeve, J.P. Ravert, P. Marmottant, and M. Versluis, Role of the chatmel geometry on the bubble pinch-off in flow-focusing devices. Physical Review Letters, 100, (2008). 

M. Hashimoto, P. Garstecki, and G.M. Whitesides, Synthesis of composite emulsions and complex foams with the use of microfluidic flow-focusing devices, Small, 3, 1792-1802, (2007). 

H.A. Stone, Formation of monodisperse bubbles in a microfluidic flow-focusing device. Applied Physics Letters, 85, 2649-2651, (2004). 

M.T. Sullivan and H.A. Stone, The role of feedback in microfluidic flow-focusing devices. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences, 366,2131-2143, (2008). 

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]. 

I. Filiz and M. Muradoglu, A Computational Study of Drop Formation in an Axisymmetric Flow-Focusing Device, Proceedings of ASME, ICNMM2006, 4th Intemahonal Coirference on Nanocharmels, Microcharmels and Minichaimels, June 19-21, Limerick, Ireland (2006). 

Fig. 1 Droplets generation in microfluidic devices, (a) Schematic channel layout of T-junction device, (b) Micrograph showing the formation of droplets in a T-junction device. Reproduced with permission from [1]. (c) Micrograph of the flow-focusing device, (d) Droplets generation at different flow conditions in flow-focusing device. Reproduced with permission from [2]...

FIGURE 20.4 A typical flow-focusing device for oil/water emulsions with characteristics dimensions. 

It is clear that using a T-junction or flow-focusing device, the breakup of the disperse phase by the continuous phase becomes periodic and predictable. The micro- or even NPs produced using microfluidics devices typically present a narrower size distribution than those produced by conventional methods,leading to consistent and regular droplets size, where control can be obtained by altering the flow rates ratio Qr of continuous and disperse phases. 

Q. Xu, M. Hashimoto, T. Dang, T. Hoare, D. Kohane, G. Whitesides, and R. Danger, Preparation of monodisperse biodegradable polymer microparticles using a microfluidic flow-focusing device for controlled drug delivery, Small, 5, 1575-1581, 2009. 

Y. Morimoto, W.-H. Tan, and S. Takeuchi, Three-dimensional axisymmetric flow-focusing device using stereolithography. Biomedical Microdevices, 11(2), 367-377, 2009. 

Bon and Kumacheva and coworkers [104] demonstrated that monodisperse solids-stabilized droplets could be generated in a microfluidic flow focusing device, whereby the solid particles were initially present in the dispersed phase. Polymerization of the monomer droplets led to hybrid polymer microspheres. They also showed that non-spherical particles could be obtained by geometric confinement of the droplets in the channel [104,105]. 

Fig. 25.11 Schematics depicting two mechanisms of droplet foimation in a microfluidic flow focusing device (FFD) (Reprinted from [43]. With permission. Copyright 2007 the Royal Society of Chemistry)...

Figure 8.22 Pictures of drop formation in flow focusing device at different values of the continuous Qoand dispersed phase g, flow rates. From [126].

M.W. Weber, R. Shandas, Computational fluid dynamics analysis of microbubble formation in microfluidic flow-focusing devices, Micrcfluidics Nanojluidics, 2007, 3, 195-206. 

M.J. Jensen, H.A. Stone, H. Bruus, A numerical study of two-phase Stokes flow in an axisymmetric flow-focusing device, Php. Fluids, 2006, 18, 077103. 

S.L. Anna, Fl.C. Mayer, Microscale tipstreaming in a microfluidic flow focusing device, Phys. Fluids, 2006, 18. 121512. 

Three different types of microsystem have been reported for the emulsification of a polymerizable liquid (Figure 18.1), namely the terrace-like microchannel device, the T-junction microchannel device and the flow focusing device (FFD). The emulsification mechanism, which is similar for these three devices, proceeds from the breakup of a liquid thread into droplets when the phase to be dispersed is sheared by the continuous and immiscible phase. 

The very first flow focusing device was developed by Stone and coworkers [11] and was used for the emulsification of water in silicone oil. The geometry of this device is depicted in Figure 18.7 and was obtained after replication of a positive relief of the microchannels patterned in SU-8 photoresist. The authors named this FFD a microfluidic flow focusing device (MFFD). A few years after the development of this microsystem, Kumacheva and coworkers [12] used an MFFD (Figure 18.7, top left) made out of PDMS or polyurethane (PU) for the emulsification and polymerization of several multifunctional acrylates ethylene glycol dimethacrylate (EGDMA),... 

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