Flow Parallel to Field or Interface Gradient

Flow plays a similar constructive role in other filtration systems and in zone refining, all being F(=)d systems. 

Perpendicular flow occurs in chromatography, countercurrent distribution, field-flow fractionation, and related methods. Below we explain the basic mechanism by which flow assumes its vital role in these separation techniques. 

The chemical potential increments of fields and interfaces provide the selective influence generally needed for separation. The selectivity exhibits itself in the form of unequal concentration distributions along the axis of the field or across the interface. However, separation along this coordinate is often subdued by the intrinsic limitations of two-phase systems or by the inherent weakness of certain fields applied to particular classes of molecules (see Chapter 8). 

Flow not only transports solutes into new regions of space, but it 

The evacuation of solute by flow is analogous to its evacuation by fields. Evacuation also plays a critical (but usually different) role in F(=) systems, as pointed out in the previous section. 

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