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Péclet number (Pe) - Nishanth Saldanha

2,481 bytes added, 16:06, 22 March 2017
Applications to Microfluidics
== Applications to Microfluidics ==
Low length and velocity scales in microfluidic setups- > Low Reynolds number. This also implies lower convective forces when compared to larger devices. As such, Pe in a microfluidic device is usually lower than 1 and as such, implying that these regimes are diffusion dominant[Squires and Quake]
 
This has an effect in making turbulence non existent in microfluidic devices. Turbulence in macro-world often leads to faster mixing -> Diffusion induced mixing is slower than covective mixing ->Slow as in different time scales (order of minutes) [Squires and Quake].
 
Thus, low Pe systems (Pe<1), have trouble getting good mixing -> If no mixing is desired, this can be optimal. However, in reaction systems, where mixing is necessary for reactions, Low Pe can be a hindrance [Squres and Quake]. While length of channels can be increased to increase Pe, this may not be optimal in all cases.
 
The molecules dissolved in the liquid also have an effect on Pe. Larger species (proteins for example) have lower diffusion constants than salt ions (by three orders of magnitude in um^2/s). These differences in diffusivities can be taken advantage of in separation systems, as shown by the H-filter by Squires and Quake. Essentially, species with lower diffusivities will not travel as far as species with higher diffusivities. Thus, separation can be achieved in a 'H' shaped channel, where a mixture will enter on the ends of the 'H' and a separation will occur such that the low mass specie will exit the bottom of the same side, while the lighter specie will traverse the middle section of the H onto the other side. Such a device can also help with buffer exchange or to separate non motile sperm from motile sperm, for example
 
T-sensors are another class of devices that take advantage of the low Pe regimes. In these systems, two pure solutions enter on the top two ends of a 'T' junction, and then mix on the middle section of the 'T'. Low Pe, means that the mixing can be seen as function of the position on the middle section ->As one moves further down, more diffusion occurs. This also allows one to clearly view the differential mixing that occurs, where the pure species are also visible. This can be helpful in many analytical tests.
 
Both devices work in intermediate Pe range (Pe around 1), but differences in diffusivities among species is critical.
 
 
At Pe>>1, mixing can occur, either due to convectively-stirred mixing or through Taylor dispersion mediated mixing.
== References ==

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