Microfluidic device dispersion

While the previous studies refer to straight channels exceptionally, microfluidic devices often comprise channels with a curvature. It is therefore helpful to know how hydrodynamic dispersion is modified in a curved channel geometry. This aspect was investigated by Daskopoulos and Lenhoff [155] for ducts of circular cross-... 

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].

The gas flow direction was from the top to bottom of the figure. No divergence is observed in the dispersion curve of the capillary, indicating that under the given conditions the dispersion of flow is small, and that this scheme is thus adequate to study the dispersion within a device of interest. This may appear unexpected, as microfluidic devices are usually assumed to exhibit laminar flow, however it can be explained by the fast lateral diffusion of individual gas molecules, which uniformly sample the whole cross section of the tube in a very short time compared with the travel time. Below each image, its projection is shown together with an independ-... 

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...

Fig. 21 Representative microfluidic device and resulting data from ATRP on a chip a image of a microfluidic device (dimensions 25 mm x 75 mm) fabricated from UV curable thiolene resin between two glass slides b reaction data for ATRP of HPMA synthesized on a chip showing the correlation of flow rate (or residence time) to reaction time and resulting conversion of monomer (M) to polymer (ln([M]o/[M]) c comparison of number average molecular mass (M ) and poly-dispersity for -butyl acrylate prepared in a traditional round bottom flask ( Flask ) and on a chip ( CRP Chip ). (Reproduced with permission from [102])...

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. 

Depending on the sign of the zeta potential, EOF can be towards either the anode or cathode, and the apparent mobility of an analyte is the sum of its electrophoretic mobility and EOF. Electroosmotic forces act near the capillary wall, which results in a flat flow profile that is less dispersive than the parabolic flow profile associated with pressure-driven flows. EOF can be beneficial since it can enable the analysis of both anions and cations in a single run. Electroosmosis has been investigated extensively as a means for producing fluid flow for various processes in microfluidic devices [227-232]. 

Many microfluidic devices involve diffusion of concentration as well as fluid flow. The velocity crossing a plane normal to the flow is seldom uniform, so the diffusion must be examined in the midst of non-uniform velocity prohles. The simplest illustration of this effect is with Taylor dispersion. 

One apparent feature of droplets generated in microfluidic devices is their mono-dispersity in size. Formation of droplets containing precursor solutions dispersed in a continuous phase and then initiation of crosslinking, polymerization or phase separation produces monodisperse microparticles of defined compositions. 

The use of narrower rounded turns for microchip-based SCCE is reported for reducing band broadening at the corners. This approach has been shown to reduce the broadening seen for turns in other microfluidic chip designs. Other, more exotic approaches to reducing dispersion at the turns of channels on microfluidic devices have been reported. One technique using a pulsed UV laser to modify the surface of plastic chips at the turn to increase the EOF by up to 4%. This technique was shown to reduce band broadening at the turns. ... 

It is clear that using a T-junction or flow-focusing device, the breakup of the disperse phase by the continuous phase becomes periodic and predictable. The micro- or even NPs produced using microfluidics devices typically present a narrower size distribution than those produced by conventional methods,leading to consistent and regular droplets size, where control can be obtained by altering the flow rates ratio Qr of continuous and disperse phases. 

The efficiency of mixing in microfluidic devices is another important feature to consider, as in hydrodynamic focusing, the fluid stream to be mixed globally described as dispersed phase, flowing along the central microfluidics channel, is confined into a narrow stream delimited by the two adjacent streams at higher flow rates. 

Similarly, Lan et al. [7] developed a one-step microfluidic method for fabricating nanoparticle-coated patchy particles. A coaxial microfluidic device was employed to produce Janus droplets composed of curable phase and non-curable phase. The results showed that nanoparticles were dispersed either in the continuous fluid or the non-curable phase fluid. The nanoparticles (30 nm or 300-500 nm) were adsorbed onto the interface between these phases, and the curable phase was solidified by UV-irradiated polymerization. Thus, the patchy microparticles asymmetrically coated by nanoparticles were synthesized. They also employed Si02, TS-1, and fluorescent polystyrene nanoparticles as the coating materials to demonstrate the validity of the method. The microfluidic approach exhibited excellent controllability in morphology, monodispersity, and size for the nanocomposites. The morphology of the particles could be controlled from less than a hemisphere to a sphere by adjusting the flow rate ratio of the two dispersed phases. The method can be applied to other nanoparticles with specific surface properties. 

Hydrodynamic dispersion of solute slugs in a straight conduit occurs due to nonuniformities in the fluid velocity across the channel cross section. On microfluidic devices, such solutal... 

The zeta potential (Q is thought to be the same as the Stem potential which is defined at the plane dividing the Stem layer and the diffuse layer of the EDL. Zeta potential is an experimentally measurable electrical potential that characterizes the EDL, and it plays an important role in many apphcations such as stability of colloidal dispersion, characterization of biomedical polymers, electrokinetic transport of particles, and capillary electrophoresis, etc. In addition, zeta potentials of the particles and the channel wall are cmcial to the design and process control of microfluidic devices. A review on measuring the zeta potential of microfluidic substrates was provided by Kirby and Hasselbrink [3]. 

An example of OCT applications in microfluidics is the investigation of laminar dispersion in a serpentine microchannel with a Y-shape inlet (Fig. 3). Transient two-fluid mixing in microfluidic devices can be clearly observed (Fig. 4) [9]. 

In addition to the MFFD, another flow focusing device was first reported by Nisisako et al. [7] for the production of polymer particles. In that device, the hydrodynamic focusing of the dispersed phase is achieved thanks to two sheath fluids coming from both sides of the microchannel in which flows the dispersed phase (Figure 18.13, top). This sheath-flow microfluidic device (SFMD), etched in a quartz glass slab, was used in conjunction with a Y-junction to emulsify Janus droplets (Figure 18.13, middle). Two differently colored solutions of isobornyl acrylate (IBA) admixed with a small amount of a thermal initiator were fed to the... 

Figure 22.30 (a) Schematic design and functional principle ofthe microfluidic device used by Nisisako et al. [39] to form double emulsions exemplified for oil droplets containing blue and red colored water droplets inside dispersed in water, (b) Photograph showing the dispersion of the oil phase with the water droplets inside the external aqueous phase, (c) Photograph ofthe formed double emulsion. The scale bars are lOOpm (b) and 50 Xm (c) [39]. 

To obtain exact fluid dynamic rules of multiphase microflows, constant interfacial tensions are highly preferred. This is easy to achieve when using purified gas/Hquid or Hquid/Hquid systems without any dissolved substances, such as surfactants or reactants. However, only a few dispersed systems can be operated stably in plastic microfluidic devices without additive, and the reason is the similar wetting properties of both phases on microcharmel wall. For example, water/alkanes are ideal systems (Bremond et al, 2008) for PDMS devices. Water/alcohols are infrequent combinations... 

---