Determination of zeta potential by non linear curve fitting method

Abstract

Fluid motion in most microfluidic systems is often achieved by electroosmotic (EO) flow due to its unique characteristics and advantages. This method allows controlled, high precision handling and pumping of small quantities of fluids through microchannels with the use of electric field. One of the characteristic parameter that governs the flow velocity is the Zeta potential. Zeta potential is the electrical potential developed at a surface which is placed in contact with a liquid due to the formation of the electrical double layer. It is difficult to be measured accurately. Current techniques to measure Zeta potential involve experiments that infer the Zeta potential indirectly, with assumptions and approximations that might induce inaccuracies.

This work presents a new and more accurate technique to determine Zeta potential by non linear curve fitting method on data obtained through current monitoring experiment. The advantage of this method is the ability to determine both the Zeta potentials of upstream and downstream electrolytes at the same time. In contrast to the conventional current monitoring method, the approximation that the Zeta potentials of both electrolytes are identical is not required. Non linear curve fitting technique also removes errors arising from uncertainties of determining the start and end point of electrolytes displacement in the displacement time method, and also linear curve approximation in the gradient method.

A mathematical model with consideration of the mismatch flow fields at the liquid/liquid interface of the upstream and downstream electrolytes during an EO displacement flow of two solutions is adapted from Gan et al (2006) for this investigation. Based on the mathematical model and Ohm’s Law, an equation relating current changes with time is then derived. Curve fitting of the equation on the data obtained from the current monitoring experiments are performed to evaluate the Zeta potentials for the two solutions. A surface conductance term is added to the equation to yield more accurate results.

Based on the above development, the Zeta potential values are found to be reasonably close to the results published by other researchers. The results are also found to be approximately similar to values obtained by conventional methods. However, surface conductivity is found to be not reliable due to the limited accuracy of the measuring device. Future work will address the device limitation and surface conductivity can then be evaluated more accurately.