Theoretical backgrounds of FFF

A non-equilibrium theory of FFF [14] was elaborated on the principles parallel to the principles of the non-equilibrium theory of chromatography [15]. The solute is displaced in a moving fluid by a combined action of the flow and the fields applied. As long as no flow occurs in the system, a concentration gradient induced by the field will be balanced by diffusion which will lead, after a certain time, to a steady state or to a quasi-equilibrium concentration, c. As soon as the flow is activated, the quasi-equilibrium will be permanently disturbed. The distribution of concentrations across the channel can be described by the relationship 

In the present coordinate system, x designates the distance from the wall at which the solute accumulates, U is a negative magnitude that characterizes the velocity of the migration induced by the field, D is the diffusion coefficient. Constant Cq is the concentration at the coordinate x = 0. On defining / = D/ U, Eqn. 1 can be written 

For the isothermal, isoviscous flow of a Newtonian liquid, between two parallel infinite planes, that is not affected by any outer field, it applies [16] that 

In a number of practical applications of FFF the approximation given by Eqn. 6 is fully justified. Hence it can be seen that the relationship between the values of R and 

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