Flow focusing

Perfectly monodisperse microbubbling by capillary flow focusing Phys. Rev. Lett. 87,... 

Zhang K., Wu Y.S., et al. Flow focusing in unsaturated fracture networks a numerical investigation. 2004 Vadose Zone Journal 3 624-633. 

Figure 1.3. Schematic principle of flow focusing emulsification.

S.L. Anna, N. Bontoux, and FI.A. Stone Formation of Dispersions Using Flow Focusing in Microchannels. Appl. Phys. Lett. 82, 364 (2003). 

Almagro, B., A.M. Ganan-Calvo, M. Hidalgo, and A. Canals. 2006. Flow focusing pneumatic nebulizers in comparison with several micronebulizers in inductively coupled plasma atomic emission spectrometry. J. Anal. At. Spectrom. 21 770-777. 

Figure 4.43 Flow focusing interdigital micromixer made of glass (by courtesy of IMM).

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. 

Anna, S.L., Bontoux, N., Stone, H.A. (2003). Formation of dispersions using flow focusing in microchannels. Appl. Phys. Lett. 82(3), 364-366. 

Malvern Sysmex SD-2000 particle counter and sizer delivers high-performance particle size analysis from 1 to 120 pm by combining electrozone sensing with hydrodynamic sheath flow focusing. 

Early work on continental heat flow focused on a linear relationship between the local values of heat flow and heat production (Birch et al, 1968) ... 

Kelemen P. B., Whitehead J. A., Aharonov E., and Jordahl K. A. (1995b) Experiments on flow focusing in soluble porous-media, with applications to melt extraction from the Mantle. J. Geophys. Res. Solid Earth 100, 475-496. 

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... 

Qualitatively, the operation of the microfluidic flow-focusing system can be described in the following way. Two immiscible phases (e.g. Nitrogen and water, or water and oil) are delivered via their inlet channels to the flow-focusing junction. In this junction, one central inlet channel, that delivers the fluid-to-be-dispersed (e.g. Nitrogen to be dispersed into bubbles) ends upstream of a small constriction (an orifice). From the sides of the central channel, two additional ones terminate upstream of the orifice. These side channels deliver the continuous fluid (e.g. aqueous solution of surfactant). It is important that these continuous phase wets the walls of the microfluidic device preferentially. Otherwise - if the fluid-to-be-dispersed - wets the walls, the resulting flows are erratic [16] and it becomes virtually impossible to form bubbles (droplets) in a reproducible and controllable process. 

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].

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) ... 

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]).

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... 

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. 

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). 

S. Takeuchi, P. Garstecki, D.B. Weibel, and G.M. Whitesides, An axisyimnetric flow-focusing microfluidic device. Advanced Materials, 17, 1067-+, (2005). 

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). 

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. 

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. 

We have recently reported [60] a DOD system in a stiff polymeric device based on an integrated microvalve [54]. This system allows for formation of both droplets and bubbles on demand (Fig. 9), in both the flow-focusing and the T-junctions, and can be made compatible with any chemistry by the virtue of its compatibility with both polymeric and glass devices. 

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). 

A M. Ganan-Calvo, Electro-flow focusing The high-conductivity low-viscosity limit. Physical Review Letters, 98, (2007). 

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]. 

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