Fluid microfluidics devices

Miniaturized fluid handling devices have recently attracted considerable interest and gained importance in many areas of analytical chemistry and the biological sciences [50], Such microfluidic chips perform a variety of functions, ranging from analysis of biological macromolecules [51, 52] to catalysis of reactions and sensing in the gas phase [53, 54], They commonly consist of channels, valves and reaction... 

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].

V. Linder, S.K. Sia, G.M. Whitesides, Reagent-loaded cartridges for valveless and automated fluid delivery in microfluidic devices. Anal. Chem. 77 (2005) 64-71. 

In the next step we are form the walls of the channels in the microfluidic device. A new, very special polymer is spin-coated on the substrate to the desired thickness. This polymer differs from the inexpensive photoresist because it comes into contact with the later fluid. Therefore, it should have a long stability it should not form cracks, should be stable against different chemicals and it should be hydrophilic or easily hydrophilized, because otherwise water will not run through the channel. Again a photo-sensitive material is used, but this time the later channel is photochemically modified. A perfect material to use is SU-8. For details see Refs. [450,451], This part can be washed away afterwards. 

The membrane technology has been tested in microfluidic devices. Normally a membrane is mounted between two chips, which make a microchannel, and fluid is allowed to pass through the membrane channel. Some papers are available on this method, which are discussed here. Hisamoto et al. [61] reviewed the application of capillary assembled microchips on PDMS as an online... 

Jacobson, S.C., Ermakov, S.V., Ramsey, J.M., Minimizing the number of voltage sources and fluid reservoirs for electrokinetic valving in microfluidic devices. 

An additional advantage of using microfluidic devices, which we do not have the space to discuss in detail here, is the absence of turbulence (Koo and Kleinstreuer, 2003). In the context of nanoparticle synthesis, turbulence gives rise to unpredictable variations in physical conditions inside the reactor that can influence the nature of the chemical product and in particular affect the size, shape, and chemical composition. In microfluidic devices, turbulence is suppressed (due to the dominance of viscous over inertial forces) and fluid streams mix by diffusion only. This leads to a more reproducible reaction environment that may in principle allow for improved size and shape control. 

In some cases, substrates and enzymes are not soluble in the same solvent. To achieve efficient substrate conversion, a large interface between the immiscible fluids has to be established, by the formation of microemulsions or multiple-phase flow that can be conveniently obtained in microfluidic devices. Until now only a couple of examples are published in which a two-phase flow is used for biocatalysis. Goto and coworkers [431] were first to study an enzymatic reaction in a two-phase flow in a microfluidic device, in which the oxidation ofp-chlorophenol by the enzyme laccase (lignin peroxidase) was analyzed (Scheme 4.106). The surface-active enzyme was solubilized in a succinic acid aqueous buffer and the substrate (p-chlorophenol) was dissolved in isooctane. The transformation ofp-chlorophenol occurred mainly at... 

In addition to conventional pressure driven flow, electrokinetic flow is also a commonly used means of transporting liquids in microfluidic devices. One type of electrokinetic flow, electroosmotic flow, relies on the presence of an electrical double layer at the solid-liquid interface. A negatively charged surface in a flow channel will attract cationic species from the fluid to form an electrical double layer at the surface. Application of an external voltage can pull those cationic species through the flow channel inducing bulk flow. The electroosmotic flow velocity can be described... 

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. 

Microfluidic Device, in 57th APS Division of Fluid Dynamics Meeting, Seattle, WA, USA (2004). 

---