Microfluidic flow devices

Fig. 2.6.10 Specialized experimental set-up for microfluidic flow dispersion measurements. Fluid is supplied from the top, flows via a capillary through the microfluidic device to be profiled and exits at the bottom. The whole apparatus is inserted into the bore of a superconducting magnet. Spatial information is encoded by MRI techniques, using rf and imaging gradient coils that surround the microfluidic device. They are symbolized by the hollow cylinder in the figure. After the fluid has exited the device, it is led through a capillary to a microcoil, which is used to read the encoded information in a time-resolved manner. The flow rate is controlled by a laboratory-built flow controller at the outlet [59, 60].

Fig. 2.6.11 Flow dispersion profiles obtained with (a) a capillary, (b) with a model microfluidic chip device containing a channel enlargement, directly connected to a capillary and (c) with the same microfluidic chip connected to a capillary via a small mixing volume. A sketch of the model microfluidic device is placed at the right side of each image, drawn to...

Wang, H.-Y., Bhunia, A. K., and Lu, C. (2006). A microfluidic flow-through device for high throughput electrical lysis of bacterial cells based on continuous DC voltage. Biosens. Bioelectron. 22,582-588. 

J.S. Rossier, P. Morier and F. Reymond, Patent Microfluidic Flow Monitoring Device, 2003, WO 2003/004160 A004161. 

Several techniques for miniaturization of simple chemical and medical analysis systems are described. Miniaturization of total analysis systems realizes a small sample volume, a fast response and reduction of reagents. These features are useful in chemical and medical analysis. During the last decade many micro flow control devices, as well as the micro chemical sensors fabricated by three dimensional microfabrication technologies based on photofabrication, termed micromachining, have been developed. Miniaturized total analysis systems (pTAS) have been studied and some prototypes developed. In microfabricated systems, microfluidics , which represent the behavior of fluids in small sized channels, are considered and are very important in the design of micro elements used in pTAS. In this chapter microfluidics applied flow devices, micro flow control devices of active and passive microvalves, mechanical and non-mechanical micropumps and micro flow sensors fabricated by micromachining are reviewed. 

Behavior of the fluids in the microfabricated channels are different from those in the millimeter scale channels. Miniaturization of micro flow devices opens a new research field, microfluidics which represents the behavior of the fluid in the micro channel [8]. Since the Reynolds number in the micro channel is usually below 200, the flow is laminar and special design concepts are necessary for the fluidic elements of mixers, reaction coils etc. in the pTAS. Some components of flow switches and fluid filters were developed using laminar flow behavior. 

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. 

All analytical instruments besides constant improvement, in recent decades are also commonly miniaturized. This is probably also one of the reasons, why miniaturized flow devices (microfluidics) are also named sensors or biosensors, similarly to e.g. microtiter plates, miniaturized mass spectrometers, electrophysiology equipment or chemical labels for imaging. It seems then that terms sensors or biosensors are used in too many meanings (perhaps because it sounds very advanced), and this process seems to be rather irreproducible. 

Next consider flow in what is called a T-sensor. Two flows come together, join, and traverse down one channel, as illustrated in Figure 10,9, This device is used in microfluidic medical devices, which are discussed further in Chapter 11, Here you will consider only the flow (which has no special utility until the convective diffusion equation is added in Chapter 11),... 

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].

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]).

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

Hilty et al. have used remote NMR with microfluidic chips to obtain profiles of gas flow in these devices . Remote detection of the NMR signal both overcomes the sensitivity limitation of NMR and enables noninvasive measurement of microfluidic flow. Although used for gases, it can be applied to liquids also. 

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