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→Applications to Microfluidics
== Applications to Microfluidics ==
[[Image:Tsensor.jpeg|150px|right|thumbnail|'''Figure 2''' Diffusion dominated mixing that occurs in a T-sensor makes it useful in florescence based analytical tests. The analyte (in
grey) diffuses through the test stream]]
The low inertial forces present within many microfluidic setups, due to low velocity and length scales, often yield low Reynolds number flows <sup></sup>: low levels of turbulence can also be expected in these flow regimes. Thus convection is not prevalent in microfluidic setups, unless they are purposely induced. Most mixing that do occur in these devices occur due to diffusion . Diffusion induced mixing is much slower than convective mixing, with mixing times in different order of magnitude. 
In units where quick mixing is not desired, such as many analytical tests or separation systems, low Pe is ideal. T-sensors, as shown in figure 2, are an example of a class of analytical devices that benefit from low Pe.
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