Cells, hydrodynamic flows

The solution flow is nomially maintained under laminar conditions and the velocity profile across the chaimel is therefore parabolic with a maximum velocity occurring at the chaimel centre. Thanks to the well defined hydrodynamic flow regime and to the accurately detemiinable dimensions of the cell, the system lends itself well to theoretical modelling. The convective-diffiision equation for mass transport within the rectangular duct may be described by... 

Miniaturisation of various devices and systems has become a popular trend in many areas of modern nanotechnology such as microelectronics, optics, etc. In particular, this is very important in creating chemical or electrochemical sensors where the amount of sample required for the analysis is a critical parameter and must be minimized. In this work we will focus on a micrometric channel flow system. We will call such miniaturised flow cells microfluidic systems , i.e. cells with one or more dimensions being of the order of a few microns. Such microfluidic channels have kinetic and analytical properties which can be finely tuned as a function of the hydrodynamic flow. However, presently, there is no simple and direct method to monitor the corresponding flows in. situ. 

Trinh et al. [399] derived a number of similar expressions for mobility and diffusion coefficients in a similar unit cell. The cases considered by Trinh et al. were (1) electrophoretic transport with the same uniform electric field in the large pore and in the constriction, (2) hindered electrophoretic transport in the pore with uniform electric fields, (3) hydrodynamic flow in the pore, where the velocity in the second pore was related to the velocity in the first pore by the overall mass continuity equation, and (4) hindered hydrodynamic flow. All of these four cases were investigated with two different boundary condi-... 

A magnetic cell separator was constructed on a Si wafer to separate cells that were labeled with paramagnetic beads (FeO nanocrystals 50 nm) from unlabeled ones. The magnetic force was generated from thin magnetized wires (10 pm wide, 0.2 pm thick) formed by depositing a cobalt-chrome-tantalum alloy in pre-etched 0.2-pm-deep trenches. These wires were parallel and were oriented at 45° to the hydrodynamic flow direction of cells [278]. 

Flow cytometry is a very versatile technique [223] which allows the analysis of more than 104 cells per second [369,370]. This high number results in statistically significant data and distributions of cell properties. Therefore, flow cytometry is a key technique to segregate biomass (into distinct cell classes) and to study microbial populations and their dynamics, specifically the cell cycle [76, 87, 116, 200, 214, 221, 295, 329, 330, 409, 418]. Individual cells are aligned by means of controlled hydrodynamic flow patterns and pass the measuring cell one by one. One or more light sources, typically laser(s), are focused onto the stream of cells and a detection unit(s) measure(s) the scattered and/or fluorescent light (Fig. 24). Properties of whole cells such as size and shape can be... 

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

The characteristics of laminar flow can allow mathematical prediction of the solution velocity and this has led to a range of hydrodynamic devices which use forced convection as a transport component under laminar flow conditions, examples include, the -> rotating disk electrode [i],-> wall jet electrode [ii], and channel flow cell (see -> flow cell). 

Most dielectrophoretic separations of cells to date have used steric-DEP-FFF. The cells are usually effectively immobilized in potential energy minima [282] near the electrodes by a combination of gravity and electrical field forces. Afterwards, the applied hydrodynamic flow forces transport those particles that are held less strongly at the electrodes. 

A number of different competitive cell-free and cell-based binding assays under static and hydrodynamic flow conditions have been used to obtain affinity data for selectin ligands. In addition to the fact that different positive controls have been used, this makes a direct comparison of reported binding affinities difficult. Therefore, in this chapter, we quote relative affinities wherever possible (Section 16.4.3). Another problem that has a negative effect on assay reliabihty has been encountered in cases where acidic ion exchange resins are used in the final step of the antagonist synthesis [170]. Small amounts of polyanions released from the resin were found to be potent selectin inhibitors, especially for P-selectin. These polyanions are difficult to remove and are not detectable by routine analysis. As a result, published assay data for P-selectin antagonists should be considered with caution. 

Equation (2) can only be used for the pressure determination if there is molecular flow in the effusion orifice of the Knudsen cell. According to Boer-boom [30] molecular flow exists as a coarse approximation for p < 3.6/r (p in Pa, r in mm), where p is the pressure in the Knudsen cell and r is the radius of the effusion orifice. The demand for a molecular flow and the effusion orifice diameter thus determines the upper limit of the range for the vapor pressure measurement. Experimental and theoretical studies on the transition range between molecular and hydrodynamic flow were recently reviewed by Wahl-beck [89] and references quoted therein. Thermodynamic studies at the limit of the Knudsen flow region are discussed by Hiipert and Gingerich [80]. 

The effect of the interparticle interactions on the electrophoretic mobihty in concentrated dispersions was theoretically studied by Levine and Neale. They used a cell model with two alternative boundary conditions at the cell boundary to describe the hydrodynamic flow the free surface model of HappeF and the zero vorticity model of Kuwabara. The results suggested that the zero vorticity model is more appropriate, because it represents in a more correct way the... 

In the study of short-lived radicals, the presence of efficient hydrodynamic flow is essential to sustain a constant supply of electroactive material to the electrode surface and hence ensure a steady flux of radicals. For this reason, Carroll adapted the in-situ cell of Allendoerfer to provide the capability of flow [54, 55]. The changes to the helical arrangement can be seen in Fig. 15. Solution is prevented from flowing in the central ESR inactive part of the cell by a complex series of baffles. The value of flow coupled to the Allendoerfer cell was shown by the electroreduction of nitromethane in aqueous conditions the radical anion of nitromethane was observed and this was shown to have a lifetime in the order of 10 ms. To data, this is the shortest radical lifetime observed. Calculations [56] suggested that such a cell should be capable of observing radicals with lifetimes of 10 5 s. 

The high sensitivity of the Allendoerfer cell makes it of great value in the detection of unstable radicals but, for the study of the kinetics and mechanism of radical decay, the use of a hydrodynamic flow is required. The use of a controlled, defined, and laminar flow of solution past the electrode allows the criteria of mechanism to be established from the solution of the appropriate convective diffusion equation. The uncertain hydrodynamics of earlier in-situ cells employing flow, e.g. Dohrmann [42-45] and Kastening [40, 41], makes such a computational process uncertain and difficult. Similarly, the complex flow between helical electrode surface and internal wall of the quartz cell in the Allendoerfer cell [54, 55] means that the nature of the flow cannot be predicted and so the convective diffusion equation cannot be readily written down, let alone solved Such problems are not experienced by the channel electrode [59], which has well-defined hydrodynamic properties. Compton and Coles [60] adopted the channel electrode as an in-situ ESR cell. 

Circulation of the electrolyte through the parallel plate cell was provided by a magnetic pump (Iwaki MD 50 R) and the electrolyte flow rate was measured with a magneto hydrodynamic flow meter (Deltaflux). 

---