Diffusion-Based Mixing

Figure 4.19 (A) Time courses of the bioluminescence at three measurement points (2, 10, 46 mm) downstream of the Y-shaped junction. The measurement was started immediately after flow stopping. (B) Typical pattern of the driving signal for alternate pulse flow. (C) Photograph of the alternate pulse flow generated by switching at 3 Hz. This frequency is selected for the visualization of thin skins, which enhances the diffusion-based mixing along the flow axis. (D) Mixing performance of the alternate pulsed flow at a frequency ranging from 1 Hz to 1 kHz. The intensity of bioluminescence is measured at points A ( ) and B ( ) [72] (by courtesy of RSC).

In a multiphase stratified flow, the interfaces between immiscible fluids have several characteristics. Firstly, the specific interfacial area can be very large just as droplet-based flow. It can for example be about 10,000 m in a microchannel compared with only 100 m for conventional reactors used in chemical processes. Secondly, the mass transfer coefficient can be very high because of the small transfer distance and high specific interfacial area. It is more than 100 times larger than that achieved in typical industrial gas-liquid reactors. Thirdly, the interfaces of a stratified microchannel flow can be treated as nano-spaces. Simulation results show that the width of the interfaces of a stratified flow is in nanometers, and that diffusion-based mixing occurs at the interface. The interface width can be experimentally adjusted by adding surfactants. Finally, reactants only contact and react with each other at the interface. Therefore, the interfaces supply us with mediums to study interfacial phenomena, diffusion-controlled interfacial reactions and extraction. 

The millisecond-mixing times are orders of magnitude faster than calculated times for diffusion based on Fick s law [68],... 

The meteorological input required in the Unified EMEP model are the 3D horizontal and vertical wind fields, specific humidity, potential temperature cloud cover, and precipitation. The transferred surface 2D fields for use in the chemical transport model are surface pressure, 2 m temperature, surface flux of momentum, sensible and latent heat, and surface stress. All variables are given in 3-h interval. Table 13.1 lists the variables and their main purposes in the EMEP model. Inside the model different boundary layer parameters like the stability, eddy diffusion, and mixing height are calculated based on MOST. 

Characteristic features of craitroUed laminar flow in tnicrofluidics devices have been utilized in many applications such as diffusion-based separation and detection, solvent extraction, mixing, and hydrodynamic focusing [255, 256]. 

The microfluidic device design and the relative flow rate of sheath and sample play important roles in hydrodynamic focusing. Lee et al. [6] proposed a theoretical model to predict the width of focused center flow inside a microfabricated flow cytometer [6]. Based on potential flow theory, they derived the equation for flow inside a planar microfabricated flow cytometer under the two-dimensional situation shown in Fig. 3a. The flow is considered laminar, and the diffusion and mixing between focused stream and sheath flows is assumed negligible. With these assumptions, conservation of mass yields... 

Vapor Sorption and Diffusion in Mixed Matrices Based on Teflon ... 

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