Microfluidic junctions

G.F. Christopher, N.N. Noharaddin, J.A. Taylor, and S.L. Anna, Experimental observations of the squeezing-to-dripping transition in T-shaped microfluidic junctions, Physical Review E, 78, (2008). 

L. Mdndetrier-Deremble and P. Tabeling, Droplet breakup in microfluidic junctions of arbitrary angles. 

Liu, H., Zhang, Y. (2009). Droplet formation in a T-shaped microfluidic junction. Journal of Applied Physics, 106, 034906. 

Figure 21.6 Controlled production of monodisperse double emulsions in a two consecutive microfluidic junctions [82, 83]. (a) Schematic illustration of two consecutive T-junctions for producing W/O/W droplets (b) formation of W/O/W emulsion with single core in a glass microchannel. The channel has a uniform depth of 100pm. Scale bar is 200pm.

Figure 6.6 Different microfluidic junctions for droplet generation and manipulation (a) T-junction at inlet (Thorsen et al., 2001) (b) T-junction at outlet (Link et al., 2004 Tan et al., 2004) (c) Sheath-flow junction (Xu and Nakajima, 2004 Nisisako et al., 2004) (d) Y-junction (Nisisako et al., 2004, 2005).

This micro mixer, named electrohydrodynamic (EHD) microfluidic mixer, comprises a simple T-channel structure (see Figure 1.5) [91]. After passing the T-junction, a bi-laminated stream is realized. Following a downstream zone for such flow establishment, a channel zone with several electrode wires on both sides of the channel is located. In this way, an electric field perpendicular to the fluid interface is generated. Thereafter, an electrode-free zone of the channel is situated for completion of the mixing initiated. 

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. 

The T-junction, depicted in Fig. 4, is one of the most common geometries used in microfluidic chips to create discrete segments of immiscible fluids. The design of the apparatus is extremely simple - a main, straight channel, that carries the continuous fluid is joined from the side, usually at a right angle, by a channel that supplies the fluid-to-be-dispersed. 

In the first demonstration of formation of monodisperse droplets in a microfluidic T-junction [9], on the basis of the experimental results on scaling of the droplet size with the rate of flow of the continuous fluid, it was hypothesized that the droplets are sheared off from the junction by the flow of the continuous fluid, similarly to the classical models of shear-driven emulsification. However, the fact that the break-up occurs in a confined geometry of the microchannels, and that the droplet growing off the inlet of the fluid-to-be-dispersed usually occupies a significant fraction of the cross-section of the main channel, suggest that the pressure drop along a growing droplet may be an important factor in the process. 

M. De Menech, P. Garstecki, F. Jousse, and H.A. Stone, Transition from squeezing to dripping in a microfluidic T-shaped junction, Journal of Fluid Mechanics, 595, 141-161, (2008). 

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]. 

As it will be briefly described in this section, the phenomenology of droplet flows in microfluidic networks can be effectively recovered by simple numerical models [39, 41-43] that assume the generic features of the multiphase microfluidic flows. These very basic models offer an insight into the qualitative features of the dynamics. As the understanding and characterization of the flow of droplets and bubbles in capillaries, junctions, comers etc. progresses, it can be expected that the same simple models, with the appropriate numeric input will able to predict the trajectories of droplets flowing in real microfluidic networks. At the end of this section we provide an example of a quantitative match between experiment and numerical simulation. 

Even the simple numerical scripts that are currently used for simulation of droplet flows in microfluidic networks can provide for quantitative match with the experimental measurements (Fig. 8). The development of an ability for a quantitative prediction of the dynamics of physical systems will demand incorporation into the scripts the details characterizing the speed of flow of droplets, the resistance that they carry, the delays at junctions and comers. 

The basic unit operation on the pressure driven laminar flow platform is the contacting of at least two liquid streams at a microfluidic channel junction (see Fig. 7). This leads to controlled difflisional mixing at the phase interface, e g. for initiation of a (bio-) chemical reaction [105]. It can also be applied for the lateral focusing of micro-objects like particles or cells in the channel [95]. The required flow focusing channel network consists of one central and two S5munetric side channels, connected at a junction to form a common outlet channel. By varying the ratio of the flow rates, the lateral width of the central streamline within the common outlet channel can be adjusted very accurately. Consequently, micro-objects suspended in the liquid flowing... 

Figure 14. Microfluidic realization of capillary electrophoresis analysis on the electrokinetic platform. (Adapted from [123], ( Agilent Technologies, Inc. 2007. Reproduced with permission, courtesy of Agilent Technologies, Inc.) After the sample has been transported to the junction area (a) it is metered by the activated horizontal flow and injected into the separation channel (b). Therein, the sample components are electrophoreticaUy separated (c) and readout by their fluorescence signal (d). The complete microfluidic CE-chip is depicted in the center.

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