Zeta is a small, simple, powerful and reliable software that can be used to calculate zeta potentials for ka>6 from electrophoretic mobilities measured at the stationary level, from apparent zeta potentials displayed by zetameter or from apparent mobilities measured at 3 different levels in a rectangular cell.

For electrophoresis at high zeta potentials and thick double layers additional braking forces must be considered. The force exerted on the particle by the excess of counterions moving in opposite direction is called retardation. Moreover, when the particle moves fast enough the centres of the particle charge and countercharge (diffuse layer) do not coincide and thus the effective electric field strength felt by the particle is lower than the external field (relaxation).

Accurate differential equations accounting for these effects have been published by O'Brien and White. These equations do not lead to accurate analytical expressions to calculate mobility from zeta potential. This problem can be solved numerically and desired accuracy can be achieved when the number of iterations is high enough. Commercial (and not open-source) programs Mobility and Winmobil are available from University of Melbourne. The solutions obtained by means of these programs are valid for any value of the zeta potential and kappa*a.

Why do we need Zeta when Mobility and Winmobil are available? For systems with kappa*a < 10 Zeta is useless. However in many systems with practical meaning 10 < kappa*a < 100 and here Zeta can help a lot (with kappa*a > 100 one can simply use Smoluchowski equation). With Mobility and Winmobil one can only calculate mobilities from zeta potentials as input data but it is mobility that can be experimentally determined. So the user has to select proper zeta potential values to cover the entire interesting range of mobilities, calculate mobilities for the selected zeta potentials and make a sort of calibration curve.

The final step is to read zeta potentials corresponding to the experimentally determined mobilities from this curve (manually or by means of some computer program).

With Zeta you simply introduce mobilities as input data and get zeta potentials as output data (no calibration curve is necessary). Unfortunately only for kappa*a > 10 the results will be reasonably accurate. This limit follows from the validity limit of the equation first published by Ohshima, H., Healy, T.W. & White, L.R., J.Colloid Interface Sci. 90, 17 (1982).

Many commercial zetameters have Smoluchowski equation built in the software, very often with the electric permitivity and viscosity data for water at 25 degrees Celsius so the results are displayed as (apparent) zeta potentials. When kappa*a < 100 the users of Winmobil need an additional step before reading from the calibration curve, namely they have to convert the apparent zeta potentials into mobilities. In Zeta, apparent zeta potentials calculated from Smoluchowski equation can be automatically converted into more accurate values if desired.