Sample inlets hydrodynamic flow

FIGURE 3.36 Schematic representation of a micromachined pre-focused 2x6 flow switch with six inlet ports and six outlet ports. The flow switch integrates two important microfluidic phenomena, including hydrodynamic focusing and valveless flow switching. The pre-focused samples can be injected into desired outlet ports precisely [263]. Reprinted with permission from the Institute of Physics Publishing. 

Beside the frit inlets for Fl-FFF (see Fig. 12) which help to enhance the sample relaxation, Moon et al. have recently suggested a frit inlet which applies a small permeable frit near the injection point in the A-Fl-FFF channel [51,401 ]. As sample materials injected into the flow streams are hydrodynamically relaxed by the compressing action of high speed frit flow, the usual focusing relaxation procedure can be avoided which makes the experiment more reliable and faster. 

A number of innovations in FFF design have been introduced recently which have improved resolution and sensitivity and reduced the major obstacles to automation. The introduction of cross-flow recirculation has resulted in better control of system pressures, thereby reducing baseline fluctuation and improving reproducibility [6]. The incorporation of a frit inlet (FI) into the flow FFF channel has permitted the use of hydrodynamic relaxation as a replacement for stop-flow relaxation, thus eliminating pressure fluctuations associated with the latter [7]. FI also reduces sample adhesion to the membrane on the accumulation wall, thereby reducing the likelihood of baseline drift and artifacts [7]. 

The laminar flow platform comprises liquid handling and (bio-) chemical assay principles, based on the stable hydrodynamic conditions in pressure driven laminar flows through microchannels. The samples are processed by injecting them into the chip inlets using external pumps or pressure sources, either batch-wise or in a continuous mode. 

In hydrodynamic focusing, a central sample solution (supplied from the middle inlet) flows within the sheath of outer fluids (supplied from the side inlets), which constrain laterally the inner sample flow to achieve a smaller stream and thinner lamination width. The extent of the width decrease of the focused stream depends on the volumetric flow rate ratio between the sample flow and sheath flows. The greater the flow rate difference, the greater the degree of width reduction. As indicated by (5), mixing time is inversely proportional to the square of the diffusion path length (in this case represented by the focused stream width), therefore. 

As the initially injected sample plug is normally a distance away from the capillary inlet in capDlaiy electrophoresis, the entrance region should have negligible influence on the species transport. In the region of fully developed (denoted by the subscript fd) flow field, the thermally induced pressure-driven flow causes additional hydrodynamic dispersion to the species diffusion. Analogous to Eq. 17, the effective dispersion coefficient is given by... 

It is seen that mixing time scales linearly with the Peclet number. However, the mixing time is decreased dramatically since W[ /. For D = 10 m /s (typical of small molecules or ions) and Wf = 100 nm, the mixing time is only 10 ps. This mixer is a very effective tool for rapidly changing the chemical environment of species in the central focused stream at the same time consuming a smaller sample volume due to the low flow rate of the inlet stream. Hydrodynamic focusing mixers find applications in the study of fast kinetics such as protein folding. 

As suggested, RTD measurements should be combined with other techniques to best quantify riser gas-phase hydrodynamics. Injection and detection methods are critical to interpreting the data. Iso-kinetic injection at different radii may help deconvolute inlet boundary conditions and flow structure. Multiple detectors along the riser length also are preferred. However, combining radial gas sampling, as practiced with steady state tracers, with radioactive impulse experiments could provide sufficient data to completely characterize riser gas-phase hydrodynamics. 

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