Theory of Mixing in Microchannels with Laminar Flow

Theory of Mixing in Microchannels with Laminar Flow 

The mixing in a microchannel takes place by diffusion and convection depending on the flow pattern and the operating conditions used [15, 16]. The final mixing on the molecular scale, where the reaction takes place, occurs only by molecular diffusion. The time for diffusion in an elementary structure is defined as 

In liquids, the value of molecular diffusion coefficient is = 10 m s . In consequence, mixing by diffusion in liquids is a very slow process. 

In the case of very low Re ( 100), a segregated flow is observed in smooth microchannels and mixing occurs by molecular diffusion. In microchannels with stratified flow and diameters of d = 100 pm, the characteristic time of diffusion is in the order of a second, which is by far too long for fast chemical reactions with characteristic reaction times less than a second. 

Under these conditions the mixing process can be promoted only by the reduc-tion of the diffusion distances. Thus, the characteristic dimension (the diameter of the channel) directly influences the micromixing time, that is, the time necessary to achieve mixing on a molecular level by diffusion (Equation 4.17). 

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Theory of Mixing in Microchannels with Laminar Flow I 137... 

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