Bistability and Electrokinetic Oscillations

Streaming potential across cation-exchange membranes in methanol–water electrolyte solutions

Streaming potential measurements across charged membranes separating two equal solutions have been carried out. Two cation-exchange membranes with different cross-linked and swelling properties (Ionics and Nafion membranes) and methanol-water electrolyte solutions of KCl have been used in the experiments. The obtained results show that the streaming potential is higher for the Ionics membrane and that the values depend on the methanol content of the solutions. A different behavior is found in the dependence of the streaming potential on the methanol percentage for each membrane. The study of the relaxation times in the decay of electrokinetic steady states of streaming potential has been carried out from the time dependence of the streaming potential when the pressure difference through the membrane is suppressed. The results show the existence of two different parts or partial relaxations, mechanical and electric. A different behavior of the mechanical relaxation time with the methanol percentage has been found for the two membranes, but any significant difference between their electric relaxation times. These differences have been explained in terms of the different degree of swelling of the membranes used.

Oscillatory phenomena at liquid–liquid interfaces

The mechanism of (i) Yoshikawa and Matsubara liquid-liquid interface oscillators and (ii) density/salt-water oscillators has been investigated. A modified simple mathematical formalism of both has been presented. Qualitative support for the model is provided by the available experimental results.

Single-capillary Teorell oscillator—studies with nonelectrolytes

It has been demonstrated that if in a Teorell oscillator, the membrane is replaced by a single capillary, the system behaves much better and becomes easily reproducible. It has also been demonstrated that oscillatory behavior in a Teorell-type setup is not limited to electrolytes; even nonelectrolytes can show oscillations. The only requirement seems to be that the nonelectrolyte should be capable of generating an electrical double layer in the capillary. This is the first report of a Teorell oscillator with a single capillary. Also, oscillations with nonelectrolytes in a Teorell-type setup have not been reported before.

Membrane oscillations involving electrokinetic phenomena

Semi theoretical models have been proposed to account for the mechanism of membrane oscillations involving, electrokinetic phenomena in systems where (i) concentration difference deltaC is finite and deltaP is varying but the current is fixed, and (ii) deltaC=0, pressure difference deltaP is fixed across the membrane and imposed current is fixed. The formalism leads to the van der Pol equation in both cases. Computer simulation has also been attempted, which indicates oscillations in the former case.

Interface-mediated oscillatory phenomena

Oscillatory transport processes which occur in the far from equilibrium region have assumed great significance from the viewpoint of science of complexity. Oscillatory phenomena in the chemical reaction systems have been subjected to intense investigations both from theoretical and experimental angles. In the present review an effort has been made to bring transport processes other than conventional chemical reactions into focus: transport processes mediated by solid-liquid and liquid-liquid interfaces have been discussed. Transport through membranes including liquid membranes, liquid-liquid interfaces and the recently reported hydrodynamic oscillator have been covered. Applications of these systems in areas such as fabrication of sensors, phase transfer catalysis and, of course, the obvious biological action, e.g. excitation of biomembranes and tissues, have been reviewed. Theoretical frameworks proposed to rationalize the phenomena have also been critically reviewed.