Interface-mediated oscillatory phenomena

Abstract

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.

Introduction

A good number of biological transport phenomena, which are rhythmical in nature are interface mediated. The exotic oscillatory phenomena, particularly those relating to excitable tissues and membranes, occur in the non-linear far from equilibrium region. A quantitative study of the oscillatory phenomena in actual biological systems, throwing light on the physics and physical chemistry of the processes is prohibitively difficult due to the chemical and structural complexities of the biological materials. This is why well-characterizable artificial systems capable of mimicking the events of excitable tissues and membranes have been experimented with. In the present article we have focused attention on physical phenomena of biological relevance. We plan to discuss some of the crucial oscillatory phenomena mediated by: (i) solid–liquid interfaces; and (ii) liquid–liquid interfaces. Although the major motivation for this article is the biological context, a few studies, which do not have an apparent biological relevance, have also been included. For example recent studies from our group have thrown new light on the phase transfer catalytic system; a discussion of these studies has also been included in this article.

There is another inherent interest in the study of oscillatory phenomena. Non-equilibrium thermodynamics have proven most useful in the study of coupled flow processes in the steady state when the linear relationships between fluxes and forces are obeyed. Since the domain of validity of Gibb's entropy equation, which is utilized for the proper choice of fluxes and forces, is larger than those of both linear phenomenological relations and Onsagar's relations, attempts have been made [1] to explore the non-equilibrium region by considering non-linear relationships between fluxes and forces. In the non-linear region beyond the domain of validity of Gibb's equation, the thermodynamics of irreversible processes cease to be applicable. The oscillatory phenomena are not obtained in the linear region; these are obtained in the non-linear region which lies in the far from equilibrium regime. Dynamics and stability theory, together with the subservient role of the thermodynamics of irreversible processes has helped in exploring the far from equilibrium region. The far from equilibrium region where the phenomena like bistability and oscillations are observed has been subjected to intense investigations in chemically reacting systems [2]. A chemically reacting system can be held at a desired distance from equilibrium and experimented with using a continuously stirred tank reactor (CSTR). The interface mediated transport processes, e.g. membrane transport, electrokinetics, etc., are the examples amongst the non-reacting systems, where also the system can be maintained at a desired distance from equilibrium by controlling the magnitude of fluxes and forces and the non-linear far from equilibrium region explored experimentally. In fact, experimental investigation of the far from equilibrium region in chemically non-reacting systems has been hindered due to non-availability of suitable experimental systems. Above all the study of oscillatory transport processes has assumed great significance from the viewpoint of science of complexity, which is considered to be the science of the 21st century [3].