Flow Focusing Geometries

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]).

FORMATION OF BUBBLES AND DROPLETS IN A PLANAR FLOW-FOCUSING GEOMETRY... 

In our experiments [15] on using the microfluidic flow-focusing geometry for formation of monodisperse bubbles of Nitrogen in a continuous liquid of aqueous solutions of surfactant and glycerin we found that the volume of the bubbles (V) depended on the pressure (p) applied to the stream of gas, the rate of flow (Q) of the continuous liquid and its viscosity (p) (Fig. 5) ... 

The usual way of feeding the microfluidic systems with fluids is to apply either a constant rate of inflow into the chip, or a constant pressirre at the inlet [20]. Formation of droplets or bubbles in systems with such, fixed, boundary conditions for flow is realtively well understood. Two microfluidic geometries are most commonly used a microfluidic T-junction [1] or a microfluidic flow-focusing geometry [6]. 

The flow focusing geometry was first introduced in an axi-s5mimetric system by Ganan-Calvo [22]. Later, the same concept was succesfiilly used in a - typical to current microfluidic techniques - planar chip by Anna et al. 

Yobas L, Martens S, Ong W, Ranganathan N (2006) High-performance flow-focusing geometry for spontaneous generation of monodispersed droplets. Lab Chip 6(8) 1073-1079. http //dx.doi.org/10.1039/B602240E... 

Molly, K. M. Jonathan, P. R. Scale-up and control of droplet production in coupled microfluidic flow-focusing geometries. Microfluidics and Nanofluidics (2012), 13(1), 65-73. 

Figure 14.3 Flow-focusing geometry implemented in a microfluidic device. An orifice is placed at a distance Hf = 161 jim downstream of three coaxial inlet streams. Water flows in the central channel, W = 197 pm, while oil flows in the two outer channels. Wo = 278 pm. The total width of the channel is W = 953 pm, and the width of the orifice is D = 43.5 pm. The thickness of the internal walls in the device is 105 pm this...

Figure 1.4 Flow instabilities relevant to multifluid systems in microchannel networks. The different miscible or immiscible phases are indicated as 1, 2, 3 and 4. (a) Breakup in a flow-focusing geometry [62, 181] due to a Rayleigh-Plateau instability. Bubble and droplet chains and multiple emulsions were prepared [71]. (b) Pressure-induced breakup [64, 72, 182]. (c) Taylor cone formation...

A two-dimensional microfabricated flow focusing geometry, containing a central channel for gas delivery, two flanking chaimels for liquid delivery and a focusing orifice is shown in Figure 8.9 [68, 69]. The typical height of the channels was 30 pm... 

I S Mixing and Contacting of Heterogeneous Systems 8 2.2.2 Flow Focusing Geometries... 

Two-dimensional (2D) microfluidic systems that can produce highly monodisperse emulsion droplets have been intensively studied in various fields [59-63]. Confined microfluidic channels such as T-junctions [64-68], cross-junctions [69-71], flow-focusing geometries [72-79] and other co-flow geometries [67, 80, 81] are generally used. Under the conditions of low Reynolds and capillary numbers [66], highly monodisperse emulsion droplets are reproducibly formed in the channels, typically... 

Figure 1.77 A triangular focusing geometry for multi lamellae flows (source IMM).

Garstecki et al. conducted careful experiments [13] in which they varied (i) the geometry of the device, (ii) the rates of flow of the two fluids, (iii) the viscosity of the continuous fluid and (iv) the value of the interfacial tension. These experimental results verified that at low values of the Capillary number - which are t5 ical to those t5 ical for flows in microsystems -indeed the mechanism of break-up is similar to that observed in the flow-focusing system. Namely, as the tip of the dispersed phase enters the main channel, and fills its cross-section, the hydraulic resistance to flow in the thin films between the interface and the walls of the obstructed microchannel creates an additional pressure drop along the growing droplet. This pressure drop has a primary influence on the d5mamics of break-up namely, once the main channel is obstructed by the growing droplet, the upstream interface of... 

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). 

W. Lee, L.M. Walker, and S.L. Anna, Role of geometry and fluid properties in droplet and thread formation processes in planar flow focusing, Physics of Fluids, 21, (2009). 

Droplet microfluidics is a science and technology of controlled formation of droplets and bubbles in microfluidic channels. The first demonstration of formation of monodisperse aqueous droplets on chip - in a microfluidic T-junction [1] - was reported in 2001. Since then, a number of studies extended the range of techniques, from the T-junction [2-5], to flow-focusing [6-10] and other geometries [11], and the capabilities in the range of diameters of droplets and their architectures [12-16]. These techniques opened attractive vistas to applications in preparatory techniques [17-19], and - what is the focus of this lecture - analytical techniques based on performing reactions inside micro-droplets. 

Figure 14.2 Schematic representations of various types of microfluidic droplet generator stream geometries, (a) Flow-focusing (b) T-junction (c) Terrace-like (d) Co-flowing. The droplet and the disperse phases are labeled as A and B, respectively. (Reprinted with permission from Ref. [6] 2011, Royal Society of Chemistry.)...

Injection mixers are similar to flow focusing mixers in that they also dilute one stream into another stream however, they differ from focusing mixers in that a small aperture is used to inject a thin stream into another stream. These mixers enable multiple flow streams to be injected and may be more desirable when flow rates or pressures cannot be controlled. Mixing time depends on the method of injection and device geometry. However, construction of small apertures or the... 

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),... 

Figure 8.6 Microfluidic geometries for droplet generation include (a) coflow, (b) T-junction, and (c) flow-focusing devices.

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